add test for cache_version precision

[lsylvester]

Adds test to ensure that the cache_version supports subseconds on databases that support it. This should help preventing losing precision when optimising the cache keys in #33835.

Note that the additional touch in test_cache_key_format_is_precise_enough was added as the test was intermittently passing depending if the test was running after first second of test suite. And the skip for test_cache_key_format_is_not_too_precise was dropped because the test is still valid for when there is no sub-second support.

/cc @schneems

[rails-bot]

r? @kaspth

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[schneems]

Nice! Instead of ensuring that the value is false we should record the original value and set it back to that.

Also instead of relying on code execution timings we should put the calls in timecop blocks to guarantee timing.

[schneems]

I think this test is fixed on master. Can you rebase?

[schneems]

There's a few code climate issues

[lsylvester]

@schneems I have fixed the code climate issues and squashed my commits. This branch should be good now.

[schneems]

Thanks!