Unit tests that wish to spawn threads for testing concurrency may now do so
by using unit_test::thread as a replacement for std::thread. These threads
propagate unhandled exceptions to the unit test, and work with the abort on
failure feature.
* Some runner member functions are now thread-safe.
* De-inline and tidy up declarations and definitions.
* arg() interface allows command lines to be passed to suites.
* Header-only!
* No external dependencies or other beast modules
* Compilation options allow for:
- Stand-alone application to run a single test suite
- Stand-alone application to run a set of test suites
- Global suite of tests inline with the host application
- Disable test suite generation completely
* Existing tests reworked to use the new classes