Add some tests for wrapper backend and skip some tests that are not supported by pocl.

1. In addition to SEGV in a lot of cases, PoCL [quit quietly when encounter unimplemented features](https://github.com/pocl/pocl/blob/master/lib/CL/pocl_cl.h#L71). This patch skip all the incorrectly behaving ones in `test_wrapper.py`
2. Add tests for a few more functions, in particular `enqueue_marker/barrier`, `wait_for_events` and `unload_platform_compiler`