Move the timing of the runnable test check to earlier in test run

Check if potential test modules are compatatible with the `riak_test` API as part of the determination of which tests to run in the `riak_test:main` function instead of doing the check in the `riak_test_runner` module. This speeds up execution of test runs by avoiding the penalty of attempting to stop all nodes for modules that are not actually runnable tests.

This is especially helpful in situations where the test involves nodes other than just riak nodes. *e.g.* riak_ee or riak_cs tests