run-tests: skip threading for a single test (issue5040)

This version backs out 50e621fe0362 and implements it in
a more consistent manner.

tests/run-tests.py

@@ -1442,6 +1442,8 class TestSuite(unittest.TestSuite):
         executes on its own thread. Tests actually spawn new processes, so
         state mutation should not be an issue.
 
+        If there is only one job, it will use the main thread.
+
         whitelist and blacklist denote tests that have been whitelisted and
         blacklisted, respectively. These arguments don't belong in TestSuite.
         Instead, whitelist and blacklist should be handled by the thing that
@@ -1558,10 +1560,6 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:
@@ -1581,6 +1579,9 class TestSuite(unittest.TestSuite):
                                     self._loadtest(test.name, num_tests[0]))
                             else:
                                 tests.append(test)
+                    if self._jobs == 1:
+                        job(test, result)
+                    else:
                         t = threading.Thread(target=job, name=test.name,
                                              args=(test, result))
                         t.start()