Add `--foo` CLI options as companions for any "var-args" options

[bobbobbio]

Use Cases Right now all the test runners have a "var-args" argument which passes extra command line arguments to the test invocation. These are accepted on the command-line only by putting extra arguments after --. It might be nice to allow the user to specify these additionally via --foo style option like --foo=--a --foo=--b etc.

We would probably accept these in addition to the ones received via -- and prepend them to the list. I'm really not sure how we represent this in the help text.

Acceptance Criteria Every test runner which accepts a "var-args" argument, accepts the same thing via a regular option like --foo provided multiple times. These new options have the same name as the "var-args" option and share the same config value.

Implementation Considerations

Definition of Done

• [ ] CHANGELOG.md updated.
• [ ] Documentation in doc/book/head updated.
• [ ] Issue is in the correct milestone.