(REF) E2E_Extern_*RestTest - Extract `isOldQSupported()`

(REF) E2E_Extern_*RestTest - Extract `isOldQSupported()`

This patch changes the way a certain carve-out works, but it doesn't change the outcome.

Background: There are two E2E tests:

* `LegacyRestTest` extends `BaseRestTest` and targets `extern/rest.php`.
  It tests `rest.php?entity=ENTITY&action=ACTION`
  as well as `rest.php?q=civicrm/ENTITY/ACTION`.
* `AuthxRestTest` extends `BaseRestTest` and targets `civicrm/ajax/rest`.
  It only tests `civicrm/ajax/rest?entity=ENTITY&action=ACTION`.

The `q` parameter cannot be used realiably with CMS routing
(`q=civicrm/ajax/rest&q=civicrm/ENTITY/ACTION`).  So we skip testing that
combination.

The patch merely changes how the skip works.