Tests: Properly check `$.uiBackCompat` in common widget tests

[mgol]

The "common widget" tests, checking if a widget doesn't overwrite some core widget APIs wasn't running as it was incorrectly checking for $.uiBackCompat === false instead of $.uiBackCompat !== true after the default changed in gh-2250.

Fixing the check uncovered that the draggable & sortable modules do overwrite the _trigger method. Add an exception in the test for that; at this stage of the project we don't plan to change the implementation.

Ref gh-2250