run-tests: only sort files when not given as argument

os.listdir returns the files in any order. This has to be sorted.
But when given as argument, the user should be allowed to set any order.

This restores the behaviour before 9848a94e2a.

tests/run-tests.py

@@ -1244,8 +1244,7 b' def main():'
         checktools()
 
         if len(args) == 0:
-            args = os.listdir(".")
-        args.sort()
+            args = sorted(os.listdir("."))
 
     tests = args