For #5757

Positional arguments are not always the last ones in the args list: in the repro case in #5757 (comment) the arg list is [ "tests", "-sv"]. This PR makes the parsing of positional args more flexible: instead of slicing the args array and keeping everything after the last option, we filter out non-positional args.

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• [n/a] Appropriate comments and documentation strings in the code
• [n/a] Has sufficient logging.
• [n/a] Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated
• [n/a] Test plan is updated as appropriate
• [n/a] package-lock.json has been regenerated by running npm install (if dependencies have changed)
• [n/a] The wiki is updated with any design decisions/details.