Add `help` option to `runCmdStanTests.py`

Summary:

Add a help option to runCmdStanTests.py. Currently, there's only usage info.

Description:

Stan's runTests.py has a help option that looks like:

$ ./runTests.py -h
usage: runTests.py [-h] [-j N] [-f F [F ...]] [-d] [-m] [--run-all]
                   tests [tests ...]

Generate and run stan math library tests.

positional arguments:
  tests            The path(s) to the test case(s) to run.
                   Example: 'test/unit', 'test/prob', and/or
                            'test/unit/math/prim/scal/fun/abs_test.cpp'

optional arguments:
  -h, --help       show this help message and exit
  -j N             number of cores for make to use
  -f F [F ...]     Only tests with file names matching these will be executed.
                   Example: '-f chol', '-f gpu', '-f prim mat'
  -d, --debug      request additional script debugging output.
  -m, --make-only  Don't run tests, just try to make them.
  --run-all        Don't stop at the first test failure, run all of them.

See more information at: https://github.com/stan-dev/math

The help should look similar.

Current Version:

v2.18.0

Start by reading runCmdStanTests.py and inspect its current usage or argument-parsing behavior. Compare it with the help output from Stan's runTests.py, then run runCmdStanTests.py -h to verify that the new help option displays usage, arguments, and exits successfully.