go test -x -ldflags "-X foo.Foo=bar" で実行

foo.go

package foo

const Foo = "fooooooo"

foo_test.go

package foo

import (
	"testing"
)

func TestSimple(t *testing.T) {
	value := Foo
	expected := "fooooooo"
	if value != expected {
		t.Fatalf("Expected %v, but %v:", expected, value)
	}
}