Random term meta test failures

[SergeyBiryukov]

There is a recent race condition in the tests, causing random failures:
​https://travis-ci.com/github/WordPress/wordpress-develop/builds/180906200
​https://travis-ci.com/github/WordPress/wordpress-develop/builds/180910443

There was 1 failure:

1) Tests_Term_Meta::test_term_meta_should_be_lazy_loaded_for_all_terms_in_wp_query_loop
Failed asserting that 201618 is identical to 201619.

/var/www/tests/phpunit/tests/term/meta.php:155

Could be a coincidence, but [48840] seems to be the only recent commit dealing with terms, so might be related.

[SergeyBiryukov]

It looks like [48840] is not the culprit per se, it just uncovered the fact that the failing test was fragile in the first place.

Specifically, the test ensures that metadata for an orphan term is not lazy loaded, because it's not attached to any posts. However, by the time the test runs, the WP_Metadata_Lazyloader queue contains quite a few IDs from previous tests, one of which can coincide with the orphan term ID from this test. When that happens, the term meta is retrieved from the cache and not from the database, as the test attempts to verify.

Clearing the lazyloader queue at the beginning of the test appears to resolve the issue.

[SergeyBiryukov]

In 48844:

Tests: Clear any previous term IDs from the metadata lazyloader queue in term meta lazy-loading test.

This fixes random test failures when one of the term IDs from previous tests coincides with the orphan term ID from this test that should not be lazy-loaded.

Follow-up to [34529], [36566].

Fixes #51109.