This does a few things to support this new behaviour:
1. Adds the ability for about:preferences category modules to indicate that
they want to specially handle subcategories. In this particular case, it
causes the "migrate" subcategory of the "general" category to open up the
migration dialog.
2. Updates MigrationUtils to open about:preferences#general-migrate if the opener
is a tabbrowser window.
3. Adds some utility functions to make writing tests easier.
4. Updates existing tests to expect the wizard to be opened in the about:preferences
subdialog.
5. Makes pressing "Escape" close the migration wizard when it's loaded in the
migration-dialog.html document.
Differential Revision: https://phabricator.services.mozilla.com/D167873
9c17617fb0
