Only reset taxonomies and post types between tests when running core tests.

A growing number of plugins and other WP-based projects use the core test
tools, such as WP_UnitTestCase, as the basis of their own tests and continuous
integration setups. At the same time, many of these third-party plugins use
custom post types and taxonomies, which are generally registered a single time
during a run of the tests: at 'init', before the testcases have run. Wiping out
these globals between tests will mess with these third-party builds.

Best practice for plugin developers is probably to clean up their own post types
and taxonomies and then reinitialize before each test. But, in the interest of
not breaking everyone's builds, the core test suite will not enforce this.

Fixes #29827.

git-svn-id: https://develop.svn.wordpress.org/trunk@29869 602fd350-edb4-49c9-b593-d223f7449a82