Change the workflow of the module's tests

Currently, the test in this module rely on the page callback in the search_api_test module in the Search API project. This is an unnecessary cross-dependency, as complex queries can only be added in a very complicated way.
Instead, we can just execute the queries directly in the test code, using search_api_query(), as long as we manually reset the caches after changing server settings.
This lets us easily add more complex tests.

In the patch attached in #1, there are also several new tests added, regarding handling of multi-valued fields.
Additionally, I have removed all unnecessary/wrong t() calls, as per the official documentation.

This new tests rely on a change in the Search API testing module. Also, it uncovered several bugs, one of them in the Search API itself. Therefore, tests with this patch will fail without fixes for the following issues:

• #2013581: Add multi-valued field to test module (Seach API)
• #2012706: Fix $reset parameter for load functions (Seach API)
• #1863672: Multiple content in the search result, when using multiple terms to filtering (still no completely working patch)
• #2012688: Fix left-over data when executing two searches in a row

Furthermore, due to the lag for the test bot regarding dependencies (#1993504: Test seems to use older version of a dependency), the automatic test of this patch will fail until there is a new stable release of the Search API with the two needed patches included. Thus, we cannot commit this patch until then.