Raise parallelism in grpc._cython._channel_test

The underlying bug in Core that this test was written to isolate was
fixed weeks ago.

src/python/grpcio/tests/unit/_cython/_channel_test.py

@@ -33,8 +33,7 @@ import unittest
 
 from grpc._cython import cygrpc
 
-# TODO(nathaniel): This should be at least one hundred. Why not one thousand?
-_PARALLELISM = 4
+from tests.unit.framework.common import test_constants
 
 
 def _channel_and_completion_queue():
@@ -61,7 +60,7 @@ def _create_loop_destroy():
 def _in_parallel(behavior, arguments):
   threads = tuple(
       threading.Thread(target=behavior, args=arguments)
-      for _ in range(_PARALLELISM))
+      for _ in range(test_constants.PARALLELISM))
   for thread in threads:
     thread.start()
   for thread in threads: