Stop checking Trac to skip tests against open tickets.

In general, skipped tests should live only in patches, which are committed at
the same time that the corresponding bug is fixed. In cases where it's
necessary to skip a test, use `markTestSkipped()` to declare this fact
explicitly.

We continue to check Trac when using `WP_UnitTestCase` to run non-core tests.

See #30284.

git-svn-id: https://develop.svn.wordpress.org/trunk@30526 602fd350-edb4-49c9-b593-d223f7449a82