Explicitly declare hierarchical=false for post_tag in create_initial_taxonomies().

In certain edge cases, such as during the automated test suite,
create_initial_taxonomies() can be called in a situation where a non-default
permastruct setting has been saved outside of wp-admin. This causes the
'hierarchical' setting for the post_tag taxonomy to remain unset, leading to
PHP notices in get_term_link().

See #29827.

git-svn-id: https://develop.svn.wordpress.org/trunk@29859 602fd350-edb4-49c9-b593-d223f7449a82
Reset post types and taxonomies before each unit test.

Registering a post type or taxonomy during a unit test causes modifications to
global variables. If the test fails to clean up these globals - either by
neglecting to call _unregister_post_type()/_unregister_taxonomy() at all or by
failing before getting a chance to do so - tests that run later in the suite
can fail, leading to much gnashing of teeth. Wiping all taxonomies and
restoring to the defaults before each test ensures that we always start with a
clean slate.

Fixes #29827.

git-svn-id: https://develop.svn.wordpress.org/trunk@29860 602fd350-edb4-49c9-b593-d223f7449a82