Add feature to dump information about tests

[ktbarrett]

Many users have (ab)used the previous detail that tests in MODULEs were Test objects that they could grab to discover tests themselves. How tests are registered was changed and their code is broken. Perhaps it's possible we can provide this functionality rather than have users depend upon implementation details and homegrown test discovery routines.

Given a MODULE and TESTCASE/COCOTB_TEST_FILTER we can dump to a JSON file the names of the tests, TestFactory/parameterize arguments, docstrings, expect_fail, timeouts, skip, whether the test would be run or not, etc. etc. We could literally serialize the Test objects used by the regression manager that are currently saved into the __cocotb_tests__ attribute in each MODULE.

This would be formatted as a different entry point that would not run the scheduler. The switch would be controlled via environment variable; maybe something like COCOTB_LIST_TESTS or COCOTB_DRY_RUN. Loading the simulator is necessary as it's common to reflect upon the design to parameterize tests.