test additions/adjustments from syncstorage-rs

[pjenvey]

I'm tracking a few changes in the feat/spanner-tests-todo branch. It's mostly just adding a new test.

We may also need to adjust the strictness of test_timestamp_numbers_are_decimals. syncstorage-rs sometimes produces only 1 decimal point vs 2 (and I doubt the client cares but we should double check before changing this test).

FAIL: test_timestamp_numbers_are_decimals (syncstorage.tests.functional.support.LiveTestCases)
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Traceback (most recent call last):
  File "syncstorage/tests/functional/test_storage.py", line 899, in test_timestamp_numbers_are_decimals
    self.assertEqual(len(str(ts).split(".")[-1]), 2)
AssertionError: 1 != 2

[pjenvey]

Another item here: fxa_uid/kid + hashed_fxa_uid are not always treated similarly alongside the legacy user_id