Add a method to clear cached variants from SearchVariant, and a configuration flag for whether
a variant should be enabled or not. Add a FullTextSearch TestState class which will globally
disable the queuedjobs and fulltextsearch shutdown handlers during tests, and is not used to
clear cached variants on each test to prevent global state leakage.
Also removes Phockito as a test dependency.
PHPUnit now includes matching functions like `any()` and `anything()` outside of a namespace, which conflicts with `Phockito` that does the same thing when calling `Phockito::include_hamcrest(true)`.
This test in particular breaks Behat, because the file includes a class (`SolrReindexTest_Item`) that extends `DataObject`, meaning that this file gets included very early in the initialisation process.
Instead, we can just set `Phockito::include_hamcrest(false)` and use the full path to the Hamcrest matching functions instead (e.g. `\Hamcrest_Matchers::anything()`).
