Avoid non-determinism in stdlib test by sorting files

For some mysterious reason I still need to investigate, some invalid
super-calls in the stdlib that should be detected by the previous commit
are only detected when the files are compiled in a specific order, this
is made more confusing by the order of listed files being
non-deterministic by default. As a first step towards sanity, sort the
files to avoid non-determinism and consistently detect the invalid
super-calls.

Author: smarter

compiler/test/dotty/tools/TestSources.scala

@@ -21,7 +21,7 @@ object TestSources {
       val acc2 = files.foldLeft(acc)((acc1, file) => if (file.isFile && file.getPath.endsWith(".scala")) file.getPath :: acc1 else acc1)
       files.foldLeft(acc2)((acc3, file) => if (file.isDirectory) collectAllFilesInDir(file, acc3) else acc3)
     }
-    collectAllFilesInDir(new File(stdLibPath), Nil)
+    collectAllFilesInDir(new File(stdLibPath), Nil).sorted
   }
 
   // pos tests lists