Add test cases for mutexing for async calls from the same thread.

Add test cases for mutexing for async calls from the same thread.

The standard Vala lock() call is a recursive lock, so it only prevents
concurrent access between different threads. But we rarely run in
distinct threads, so this test uses a more-complete solution that we can
depend upon (since the test will fail if its assumptions ever change in
Vala).

Helps: bgo#652637 - Don't hold locks across async calls