Issue 15302 switched regrtest from getopt to argparse for parsing options.  However, regrtest.main() still expects and operates on getopt-style options.

This issue is to continue the regrtest refactoring and replace the use of getopt-style options with an argparse Namespace object.

This issue should probably be a meta-issue with the transition happening over several issues/commits, as there are many command-line options that will probably have varying types and actions (in the sense of argparse).  Options can be switched over incrementally in groups (e.g. by having the _parse_args() function return the parsed options in both forms: in both getopt-style format and a Namespace object).

This issue will be completed when the namespace-to-getopt "bridge code" is removed -- probably along with the corresponding argparse-to-getopt tests.