[test] Make coverage in test-runner tests work with multiprocessing

This replaces multiprocessing with threading in tests to make python
coverage work.

TBR=sergiyb@chromium.org

Bug: v8:6917
Change-Id: Idff763dfefa4a7fc782133d94089b3a5b00a194d
Reviewed-on: https://chromium-review.googlesource.com/844735
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50344}

tools/testrunner/local/pool.py

@@ -8,6 +8,21 @@ from multiprocessing import Event, Process, Queue
 import traceback
 
 
+def setup_testing():
+  """For testing only: Use threading under the hood instead of multiprocessing
+  to make coverage work.
+  """
+  global Queue
+  global Event
+  global Process
+  del Queue
+  del Event
+  del Process
+  from Queue import Queue
+  from threading import Event
+  from threading import Thread as Process
+
+
 class NormalResult():
   def __init__(self, result):
     self.result = result

tools/unittests/run_tests_test.py

@@ -122,14 +122,12 @@ class SystemTest(unittest.TestCase):
     try:
       import coverage
       if int(coverage.__version__.split('.')[0]) < 4:
-        # First coverage 4.0 can deal with multiprocessing.
         cls._cov = None
         print 'Python coverage version >= 4 required.'
         raise ImportError()
       cls._cov = coverage.Coverage(
           source=([os.path.join(TOOLS_ROOT, 'testrunner')]),
           omit=['*unittest*', '*__init__.py'],
-          concurrency='multiprocessing',
       )
       cls._cov.exclude('raise NotImplementedError')
       cls._cov.exclude('if __name__ == .__main__.:')
@@ -145,12 +143,13 @@ class SystemTest(unittest.TestCase):
     sys.path.append(TOOLS_ROOT)
     global standard_runner
     from testrunner import standard_runner
+    from testrunner.local import pool
+    pool.setup_testing()
 
   @classmethod
   def tearDownClass(cls):
     if cls._cov:
       cls._cov.stop()
-      cls._cov.combine()
       print ''
       print cls._cov.report(show_missing=True)