[D7] When adding a new menu link, restrict the available parents to the current menu

Uploading a backport of the D10 patch. I think this should be pretty safe, because when you click "add link" in a specific menu, you want to add menu link to that specific menu, so you do not need other menus displayed. Other use-cases (like editing the menu item, or using vertical tabs in node to set a menu item) are not affected by the patch.

I had to duplicate the assertNoOption() and assertOption() functions in tests, which is not ideal. But not sure if we are going to refactor the original ones (as proposed in the @todo) until the D7 EOL, so this is probably not a big issue and should not block this issue.

Adding a tag for the review from another D7 maintainer before commit.

I have drafted a CR here: https://www.drupal.org/node/3406314 (it is similar to the D10 CR)