Migrate meant to aid upgrading to jQuery n should have tests run on jQuery n-1

[mgol]

We should verify that Migrate meant to aid migration from jQuery n-1 to n passes as much as possible from the jQuery n-1 test suite. One idea is to commit the test/ directory from the Core repo's proper branch and comment out or QUnit.skip tests that are broken and we don't know how to quickly fix them (maybe creating issues for those that are possible but hard to fix). That way we'd at least know we don't regress; currently the test coverage of the whole jQuery + Migrate combination is pretty low.

cc @dmethvin & @gibson042 which discussed this idea with me on IRC.

[Krinkle]

Are there known cases of tests from jQuery N-1 not passing with jQuery N + jQuery Migrate N?

[mgol]

@Krinkle Answering a few years later... 😅

Are there known cases of tests from jQuery N-1 not passing with jQuery N + jQuery Migrate N?

Plenty! Migrate doesn't patch everything as the goal it not completely revert to the previous behavior, foregoing any improvements. For example, Migrate 3.x doesn't make Deferred's pipe identical to then again.

There are also support test results, some tests skipped based on those support tests, etc. - these are possible to handle but would require doing it explicitly.