Fixes #372.

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.

Steps to verify

Apply the following patch to the code, disable capturing test output (-s option to the py.test command).

diff --git google/cloud/pubsub_v1/publisher/_batch/thread.py google/cloud/pubsub_v1/publisher/_batch/thread.py
index 36dd3b9..c3aacdf 100644
--- google/cloud/pubsub_v1/publisher/_batch/thread.py
+++ google/cloud/pubsub_v1/publisher/_batch/thread.py
@@ -211,6 +211,7 @@ class Batch(base.Batch):
         commit_thread = threading.Thread(
             name="Thread-CommitBatchPublisher", target=self._commit
         )
+        print(f"batch starting commit for {len(self._messages)} message(s)")
         commit_thread.start()

     def _commit(self):

Run system tests in --verbose mode and check the output.

Actual result (before the PR fix)
The system test test_streaming_pull_blocking_shutdown fails with a high probability.

Expected result

The system test test_streaming_pull_blocking_shutdown consistently passes.

In addition, wherever the _publish_messages(..., batch_sizes=[...]) helper is used in the tests, the output size output should match the values passed through batch_sizes. For example, in test_streaming_pull_max_messages(), batch_sizes is set to (7, 4, 8, 2, 10, 1, 3, 8, 6, 1), and the test output matches that:

...
tests/system.py::TestStreamingPull::test_streaming_pull_max_messages batch starting commit for 7 message(s)
batch starting commit for 4 message(s)
batch starting commit for 8 message(s)
batch starting commit for 2 message(s)
batch starting commit for 10 message(s)
batch starting commit for 1 message(s)
batch starting commit for 3 message(s)
batch starting commit for 8 message(s)
batch starting commit for 6 message(s)
batch starting commit for 1 message(s)
...

PR checklist:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)