test: fix flaky test for blocking pull shutdown (#378)

If a test is run in a suite with other system tests, the messages
are not always published in batch sizes as desired, which can
affect how ACKs are handled on the backend (the server requires
all messages published in a single batch to be ACK-ed in order
to accept the ACKs).

If a publisher client instance is shared between the tests, the
batching can apparently be affected, thus we create a new client
instance before each test.

Since these tests are slow system tests, the overhead should not
be significant.

tests/system.py

@@ -44,12 +44,12 @@ def project():
     yield default_project
 
 
-@pytest.fixture(scope="module")
+@pytest.fixture()
 def publisher():
     yield pubsub_v1.PublisherClient()
 
 
-@pytest.fixture(scope="module")
+@pytest.fixture()
 def subscriber():
     yield pubsub_v1.SubscriberClient()