Deflake RewriteContextTest (which likely became flakey because RewriteDriver is overholding rewrite_mutex slightly less)
Deflake RewriteContextTest (which likely became flakey because
RewriteDriver is overholding rewrite_mutex slightly less)

There were 2 kinds of failures:
- In TrimRepeatedOptimizableDelayed, whether we hit a repeated
  rewrite or just 2 cache hits is non-deterministic, so relax the
  test to reflect that.
- In a bunch of invalidation tests, whether the re-fetch of input
  would be reported as an insert of indentical reinsert depended on
  the extent to which the mock clock moved, as that could change the
  Date: header. Ensure the clock moves to produce determinism.


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