run-tests: skip threading for a single test

Threading is incompatible with most Python debuggers,
which makes debugging run-tests.py a real pain.

If there is only one test to run, skip using a thread for it.

Note that --debug is not compatible with debugging tests,
since it bypasses the output handling, which is where
much of the excitement is.

tests/run-tests.py

@@ -1558,6 +1558,10 class TestSuite(unittest.TestSuite):
             statthread.start()
 
         try:
+            if len(tests) == 1:
+                test = tests.pop(0)
+                test.run(result)
+            else:
             while tests or running:
                 if not done.empty() or running == self._jobs or not tests:
                     try: