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.