Now all tests work even if Context::context, Context::device,
Context::queue are not default context, device and queue. This
is required for developing better tests in the future.
Note: Some tests may work only for default context/queue/device
since classes that they test work only for default context/q/d.
There are two solutions for this problem: either those tests run
on default queue (no matter what) or they does not run when
Context::context is not the default context. See test_string.cpp.
