Implement process isolation support for tests

Major features that could be implemented afterwards:

1. Catch should not go crazy if the test case uses `fork` or similar system call
2. Catch should allow running tests in isolated processes
3. Catch should allow testing for abort -- there should be a macro along the lines of `REQUIRE_ABORT(expr)`, that launches separate process to check `expr` and succeeds if the launched process aborts
4. Catch should allow limited time for test suite execution