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

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.

Closes gh-2286
Ref gh-2250

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