Fixing flaky schema test (hopefully!).
Fixing flaky schema test (hopefully!).

We just can't stop replication on a slave tablet while vttablet is
running. Its health check will re-enable it. And then weird behavior
ensues, as we're not in a state we expect.

So before we reset replication, we need to kill the tablets. Doing that
in schema.py test by starting and stopping the tablets for each test.

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