Add constant for PlatformNotReady wait time to use in tests

[mdegat01]

Proposed change

When a platform returns a PlatformNotReady exception then entity_platform.py handles it by waiting for 30 seconds * number of setup attempts before trying again. When testing this behavior in tests the recommendation is to try and setup the platform while forcing a failure, fix the failure, advance time, and see that is has been setup.

Since "30 seconds" was simply hard-coded into the logic tests are currently forced to advance time an arbitrarily long amount of time to ensure it was enough to satisfy this wait time (I saw code adding a few minutes generally). Thought it would be preferable to refactor this into a constant so tests can advance time by exactly that.

Also adjusted the InfluxDB sensor tests to import this new constant instead of creating it itself now that it is available. Just since that was the test I was working on that made me want this :)

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[probot-home-assistant]

Hey there @fabaff, mind taking a look at this pull request as its been labeled with an integration (influxdb) you are listed as a codeowner for? Thanks!
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