Test case `test_phuzzer.py` raises exceptions on import

https://github.com/angr/phuzzer/blob/cf9feabead99ba25882cf9db49663f2cb22d6adc/tests/test_fuzzer.py#L17

In certain cases, this will be raised causing the module not to import, which causes a ModuleLoadException failure in the CI pipeline.

My proposed solution is to replace the checks in-test with some system to determine which phuzzer backends are available, and then to refactor the tests to use`@unittest.skipUnless` to automatically skip tests not able to be run with the current configuration.