Added policyTags test for bigquery table schema customizediff func

[modular-magician]

The behavior here is correct but this is an edge case we could accidentally run into in the future.

This is a follow-on to GoogleCloudPlatform/magic-modules#4832, which was for a bug in bigquery table schema's diffsuppress func triggered by policyTag usage.

I also moved the test definition to be close to the test case definition; it seems to have drifted away over time. I think placing it after the test cases will make things less likely to be put between them in the future.

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Derived from GoogleCloudPlatform/magic-modules#4844