Register sigint handler on test execution not on import

We register a signint handler on import of openhtf here: https://github.com/google/openhtf/blob/master/openhtf/__init__.py#L76

Test.handle_sig_int does any okay job of just raising a KeyboardInterrupt if a test isn't actually running.

Would probably be better if we only registered this signal handler on test execution. I sometimes import openhtf to re-utilize some other utilities or plugs without using the majority of the framework and in these cases it's a little odd that openhtf is handling the sigint.