Making regression test put()'s transactional.

[dhermes]

This is to address flaky test failures.

See #535 for more discussion.

[coveralls]

Coverage remained the same at 100.0% when pulling e407a68 on dhermes:regression-transactional-puts into 1bfa469 on GoogleCloudPlatform:master.

[tseaver]

LGTM

[dhermes]

The second time regression tests were run after this PR, the flake still occurred:
https://travis-ci.org/GoogleCloudPlatform/gcloud-python/builds/47970589

======================================================================
FAIL: test_save_key_self_reference (datastore.TestDatastoreSaveKeys)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/GoogleCloudPlatform/gcloud-python/regression/datastore.py", line 158, in test_save_key_self_reference
    self.assertEqual(len(stored_persons), 1)
AssertionError: 0 != 1

The code at question is

        with datastore.Transaction():
            datastore.put([entity])

        query = datastore.Query(kind='Person')
        query.add_filter('linkedTo', '=', key)

        stored_persons = list(query.fetch(limit=2))
        self.assertEqual(len(stored_persons), 1)

@pcostell does the runQuery request also need to occur transactionally to make sure this passes always?

[pcostell]

This doesn't actually do anything (a single put by itself is transactional).

To make sure these tests always pass you need to make the entity have a parent then make the query do an ancestor query.

[dhermes]

Thanks. Sorry for the n00b question. I've been told that before, totally should not have forgotten.

@tseaver I'll jump on fixing this.

[pcostell]

No worries, it's easy to mix up.