Avoid NPE in ParallelTesting on junk files

Emacs often produces temporary files in directories. These used to
cause NPEs in the new testing framework. We now fix this by only
compiling file names that designate source files.

Author: odersky

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

@@ -920,9 +920,8 @@ trait ParallelTesting { self =>
   private def compilationTargets(sourceDir: JFile): (List[JFile], List[JFile]) =
     sourceDir.listFiles.foldLeft((List.empty[JFile], List.empty[JFile])) { case ((dirs, files), f) =>
       if (f.isDirectory) (f :: dirs, files)
-      else if (f.getName.endsWith(".check")) (dirs, files)
-      else if (f.getName.endsWith(".flags")) (dirs, files)
-      else (dirs, f :: files)
+      else if (isSourceFile(f)) (dirs, f :: files)
+      else (dirs, files)
     }
 
   /** Gets the name of the calling method via reflection.