Improvements to beast::unit_test framework:
* 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.
Update unit testing command line parser parameters:
A string passed by the '--unittest-arg' command line parameter is passed to
suites when unit tests run and can be used to customize test behavior.
* Add '--unittest-arg' command line argument
* Remove obsolete '--unittest-format' command line argument
Add 'sample' npm test:
test/sample-test.js is the smallest possible npm test.
Stoppable fortified:

*  Test written to confirm the current behavior.
*  Comments updated to reflect the current behavior.
*  Public API reduced to what is currently in use.
*  Protected data members made private.
*  volatile bool members changed to std::atomic<bool>.
*  std::atomic<int> members changed to std::atomic<bool>.