Respect `wp_suspend_cache_addition()`.

When `wp_suspend_cache_addition()` returns true, it should not allow new
objects to be added to the cache.
Add tests for `wp_suspend_cache_addition()`.
Make sure to disable additions in test case.
Test re-enabling cache additions.
Bump WordPress test versions.
Use WP 3.7 tests for 3.6 and 3.7 tests.
Add missing fi to bash script.
Remove reserved variable name.
Remove 3.6 and 3.7 tests and add 3.9 tests.
Merge branch 'add-suspend-cache-addition'