Merge search_api_test and search_api_test_backend modules

The search_api_test_backend module is already a bit of a mess, since it contains data types not properly namespaced with the module name. It's also used in a lot of tests that don't really test the backend.

In #2574589-3: Move the "remove unloadable items from tracking" logic to the index I'm proposing a patch that adds a search_api_test module which provides other plugins like the test backend, which allow easily controling a plugin's methods from within a test case. It should be simple to use the same mechanism (traits) also for the test backend, and just merge the two modules to one. (The data type plugins should then also be changed to have the proper namespace, i.e., start with search_api_test.)

The task is "Novice" in the sense that it's just moving around of and according slight adaptions to existing code – some knowledge of the Search API and D8 is still probably very helpful.
Also, it's probably quite a bit of work to make sure all the tests using the test backend are updated accordingly.