Help: Clarify pass, fail and skip test properties and exit code

The behavior when system-level failures occur was not previously defined.
Clarify the behavior and provide an example for how to work around them.
Affected test properties:

- PASS_REGULAR_EXPRESSION
- FAIL_REGULAR_EXPRESSION
- SKIP_REGULAR_EXPRESSION
- SKIP_RETURN_CODE

Also update the existing WILL_FAIL test property docs to using the same
consistent wording.