Make runner support single threaded mode

src/python/grpcio_tests/commands.py

@@ -127,7 +127,7 @@ class TestAio(setuptools.Command):
         import tests
         loader = tests.Loader()
         loader.loadTestsFromNames(['tests_aio'])
-        runner = tests.Runner()
+        runner = tests.Runner(dedicated_threads=False)
         result = runner.run(loader.suite)
         if not result.wasSuccessful():
             sys.exit('Test failure')

src/python/grpcio_tests/tests/_runner.py

@@ -117,8 +117,15 @@ class AugmentedCase(collections.namedtuple('AugmentedCase', ['case', 'id'])):
 
 class Runner(object):
 
-    def __init__(self):
+    def __init__(self, dedicated_threads=True):
+        """Constructs the Runner object.
+        
+        Args:
+          dedicated_threads: A bool indicates whether to spawn each unit test
+            in separate thread or not.
+        """
         self._skipped_tests = []
+        self._dedicated_threads = dedicated_threads
 
     def skip_tests(self, tests):
         self._skipped_tests = tests
@@ -194,6 +201,7 @@ class Runner(object):
                 sys.stdout.write('Running       {}\n'.format(
                     augmented_case.case.id()))
                 sys.stdout.flush()
+                if self._dedicated_threads:
                     case_thread = threading.Thread(
                         target=augmented_case.case.run, args=(result,))
                     try:
@@ -212,6 +220,8 @@ class Runner(object):
                     sys.stdout.flush()
                     result_out.truncate(0)
                     check_kill_self()
+                else:
+                    augmented_case.case.run(result)
         result.stopTestRun()
         stdout_pipe.close()
         stderr_pipe.close()
@@ -223,3 +233,4 @@ class Runner(object):
         with open('report.xml', 'wb') as report_xml_file:
             _result.jenkins_junit_xml(result).write(report_xml_file)
         return result
+