Enable compilation tests using parallel backend

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -18,6 +18,7 @@ import scala.concurrent.duration._
 import TestSources.sources
 import reporting.TestReporter
 import vulpix._
+import dotty.tools.dotc.config.ScalaSettings
 
 class CompilationTests {
   import ParallelTesting._
@@ -273,7 +274,6 @@ class CompilationTests {
     compileFilesInDir("tests/init/neg", options).checkExpectedErrors()
     compileFilesInDir("tests/init/pos", options).checkCompile()
     compileFilesInDir("tests/init/crash", options.without("-Xfatal-warnings")).checkCompile()
-
     // The regression test for i12128 has some atypical classpath requirements.
     // The test consists of three files: (a) Reflect_1  (b) Macro_2  (c) Test_3
     // which must be compiled separately. In addition:
@@ -295,6 +295,38 @@ class CompilationTests {
       tests.foreach(_.delete())
     }
   }
+
+  // parallel backend tests
+  @Test def parallelBackend: Unit = {
+    given TestGroup = TestGroup("parallelBackend")
+    val parallelism = Runtime.getRuntime().availableProcessors().min(16)
+    assumeTrue("Not enough available processors to run parallel tests", parallelism > 1)
+
+    val options = defaultOptions.and(s"-Ybackend-parallelism:${parallelism}")
+    def parCompileDir(directory: String) = compileDir(directory, options)
+
+    // Compilation units containing more than 1 source file
+    aggregateTests(
+      parCompileDir("tests/pos/i10477"),
+      parCompileDir("tests/pos/i4758"),
+      parCompileDir("tests/pos/scala2traits"),
+      parCompileDir("tests/pos/class-gadt"),
+      parCompileDir("tests/pos/tailcall"),
+      parCompileDir("tests/pos/reference"),
+      parCompileDir("tests/pos/pos_valueclasses")
+    ).checkCompile()
+
+    aggregateTests(
+      parCompileDir("tests/neg/package-implicit"),
+      parCompileDir("tests/neg/package-export")
+    ).checkExpectedErrors()
+
+    aggregateTests(
+      parCompileDir("tests/run/decorators"),
+      parCompileDir("tests/run/generic")
+    ).checkRuns()
+
+  }
 }
 
 object CompilationTests extends ParallelTesting {