build.sbt.diff

-Test / javaOptions += "-Dtest-database-name=sample"
+concurrentRestrictions := {
+  val par = parallelExecution.value
+  val max = EvaluateTask.SystemProcessors
+  List(
+    Tags.limitAll(if (par) max else 1),
+    Tags.limit(Tags.ForkedTestGroup, 4), // ここ増やした
+    Tags.exclusiveGroup(Tags.Clean)
+  )
+}
+
+Test / testGrouping := {
+  val groupCount = 4
+  def groupByHash(tests: Seq[TestDefinition]): Seq[Tests.Group] =
+    tests
+      .groupBy(x =>
+        // overflow避けるために一度Longにしている
+        scala.util.hashing.MurmurHash3
+          .stringHash(x.name)
+          .toLong
+          .abs
+          .toInt % groupCount
+      )
+      .map { case (hash, tests) =>
+        println((hash, tests.size))
+        val options = ForkOptions().withRunJVMOptions(
+          (Test / javaOptions).value.toVector ++ Vector(
+            s"-Dtest-database-name=${hash}"
+          )
+        )
+        new Tests.Group(hash.toString, tests, Tests.SubProcess(options))
+      }
+      .toSeq
+  groupByHash((Test / definedTests).value)
+}