Testdata wrapper with mutator function

testdata.patch

diff --git a/pkg/transfer/bag_validation_test.go b/pkg/transfer/bag_validation_test.go
index 578f0b29..8f48247e 100644
--- a/pkg/transfer/bag_validation_test.go
+++ b/pkg/transfer/bag_validation_test.go
@@ -2,20 +2,21 @@ package transfer_test
 
 import (
 	"os"
+	"path/filepath"
 	"testing"
 
 	"gotest.tools/v3/assert"
-	tfs "gotest.tools/v3/fs"
 
 	"gitlab.artefactual.com/clients/ste/sdps_preprocessing/pkg/transfer"
 	"gitlab.artefactual.com/clients/ste/sdps_preprocessing/pkg/wfdef"
+	"gitlab.artefactual.com/clients/ste/sdps_preprocessing/testdata"
 )
 
 func TestBagValidation(t *testing.T) {
 	t.Parallel()
 
 	type params struct {
-		transferDir    string
+		sample         *testdata.SampleTransfer
 		newTransferErr string
 		wdf            wfdef.Version
 	}
@@ -30,13 +31,47 @@ func TestBagValidation(t *testing.T) {
 	}{
 		"No errors or warnings in transfer": {
 			params: params{
-				transferDir: "../../testdata/9999---no-errors-or-warnings-wfdf-1.0---Bag---IPLv3c/",
-				wdf:         wfdef.Version("1.0"),
+				sample: testdata.NewSampleTransfer(
+					"../../testdata/9999---no-errors-or-warnings-wfdf-1.0---Bag---IPLv3c", nil,
+				),
+				wdf: wfdef.Version("1.0"),
 			},
 			ex: expected{
 				transferErrors: transferErrors(t),
 			},
 		},
+		"MC error on unexpected sub-directories": {
+			params: params{
+				sample: testdata.NewSampleTransfer(
+					"../../testdata/9999---no-errors-or-warnings-wfdf-1.0---Bag---IPLv3c",
+					func(t *testing.T, path string) {
+						// Add unexpected sub-directories in master and access.
+						dataDir := filepath.Join(path, "no-errors-or-warnings", "data")
+						assert.NilError(t, os.Mkdir(filepath.Join(dataDir, "access", "extra1"), ModeDir))
+						assert.NilError(t, os.Mkdir(filepath.Join(dataDir, "master", "extra2"), ModeDir))
+					},
+				),
+				wdf: wfdef.Version("1.0"),
+			},
+			ex: expected{
+				transferErrors: &transfer.TransferErrors{
+					Errors: map[transfer.ErrorType][]*transfer.TransferError{
+						transfer.ErrorTypeMandatoryCritical: {
+							{
+								ErrType: transfer.ErrorTypeMandatoryCritical,
+								Message: "there are unexpected folders or files",
+								Details: []string{"file path: no-errors-or-warnings/data/access/extra1"},
+							},
+							{
+								ErrType: transfer.ErrorTypeMandatoryCritical,
+								Message: "there are unexpected folders or files",
+								Details: []string{"file path: no-errors-or-warnings/data/master/extra2"},
+							},
+						},
+					},
+				},
+			},
+		},
 	}
 
 	for name, tc := range testCases {
@@ -45,7 +80,7 @@ func TestBagValidation(t *testing.T) {
 		t.Run(name, func(t *testing.T) {
 			t.Parallel()
 
-			tBag, err := transfer.NewBagTransfer(tc.params.transferDir, tc.params.wdf)
+			tBag, err := transfer.NewBagTransfer(tc.params.sample.Path(t), tc.params.wdf)
 
 			if tc.params.newTransferErr != "" {
 				assert.ErrorContains(t, err, tc.params.newTransferErr)
@@ -60,47 +95,4 @@ func TestBagValidation(t *testing.T) {
 			assert.DeepEqual(t, tc.ex.transferErrors, transferErrors)
 		})
 	}
-
-	t.Run("MC error on unexpected sub-directories", func(t *testing.T) {
-		t.Parallel()
-
-		td := tfs.NewDir(t, "test",
-			tfs.FromDir(
-				"../../testdata/9999---no-errors-or-warnings-wfdf-1.0---Bag---IPLv3c/",
-			),
-		)
-
-		// Add unexpected sub-directories in master and access.
-		for _, d := range []string{
-			td.Join("no-errors-or-warnings", "data", "access", "extra1"),
-			td.Join("no-errors-or-warnings", "data", "master", "extra2"),
-		} {
-			if err := os.Mkdir(d, ModeDir); err != nil {
-				t.Errorf("add test Bag dir: %v", err)
-			}
-		}
-
-		b, err := transfer.NewBagTransfer(td.Path(), wfdef.Version("1.0"))
-		assert.NilError(t, err)
-
-		got := transfer.NewValidator()
-		b.Validate(got)
-
-		assert.DeepEqual(t, got, &transfer.TransferErrors{
-			Errors: map[transfer.ErrorType][]*transfer.TransferError{
-				transfer.ErrorTypeMandatoryCritical: {
-					{
-						ErrType: transfer.ErrorTypeMandatoryCritical,
-						Message: "there are unexpected folders or files",
-						Details: []string{"file path: no-errors-or-warnings/data/access/extra1"},
-					},
-					{
-						ErrType: transfer.ErrorTypeMandatoryCritical,
-						Message: "there are unexpected folders or files",
-						Details: []string{"file path: no-errors-or-warnings/data/master/extra2"},
-					},
-				},
-			},
-		})
-	})
 }
diff --git a/testdata/testdata.go b/testdata/testdata.go
new file mode 100644
index 00000000..debf4668
--- /dev/null
+++ b/testdata/testdata.go
@@ -0,0 +1,46 @@
+package testdata
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/otiai10/copy"
+	"gotest.tools/v3/assert"
+)
+
+type SampleMutator = func(t *testing.T, path string)
+
+type SampleTransfer struct {
+	name    string
+	path    string
+	mutator SampleMutator
+}
+
+func NewSampleTransfer(path string, mutator SampleMutator) *SampleTransfer {
+	return &SampleTransfer{
+		name:    filepath.Base(path),
+		path:    path,
+		mutator: mutator,
+	}
+}
+
+func (st *SampleTransfer) Path(t *testing.T) string {
+	if st.mutator == nil {
+		return st.path
+	}
+
+	tmp, err := os.MkdirTemp("", "test_*")
+	assert.NilError(t, err, "Cannot create temporary directory.")
+
+	// E.g.: /tmp/test_3088682795/9999---no-errors-or-warnings-wfdf-1.0---Bag---IPLv3c.
+	dest := filepath.Join(tmp, st.name)
+
+	err = copy.Copy(st.path, dest)
+	assert.NilError(t, err, "Cannot copy sample transfer into temporary directory.")
+
+	// Run user-provided mutator.
+	st.mutator(t, dest)
+
+	return dest
+}