warnings: warning filters set by collected code during collection have no effect during test run

[bluetech]

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In 2017 there was an issue #2430 complaining that module-level calls to warnings.filterwarnings are getting reset by pytest and thus have no effect. PR #2445 added a fix for this and a test testing/test_warnings.py::test_works_with_filterwarnings (still present).

In 2018 there was a change to add warning capture during collection 1a9d913ee1f9d1a56448f659649c96214f6d9645. This means collection is now wrapped in a with warnings.catch_warnings and so all warnings.filterwarnings get reset after collection, regressing the issue #2430.

The tests continued to pass due to how it was written, however running pytest with -Wdefault causes it to (correctly) fail (see #13480).

Since we haven't received any complaints since it regressed, and a fix I think would be difficult, IMO we should just delete the test, basically WONTFIXing #2430.