Fix useSubscription executes skipped subscription if input changes

[levrik]

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

This bug triggered a regression in our application after updating from Apollo Client 3.3.21 to 3.5.7.

The unit test testing the skip flag wasn't actually working correctly. It didn't fail when skip was set to false.
I've updated it to make sure no subscription is being executed by spying on onSetup callback of the link instead of the onSubscriptionData one on the hook which isn't called anyway since simulateResult isn't called as part of the test. I also extended the test to make sure no subscription is being executed when variables are changing.
The final version of the test was verified to fail on main branch.

I wasn't sure if I shall extend the existing test or add a new one. Let me know if a new test case for the update case is preferred.

[apollo-cla]

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[levrik]

Rebased on main branch

[brainkim]

@levrik This is wonderful, though I might need to double check the useEffect() logic once more before merging...

[benjamn]

Thanks for this @levrik!

@brainkim If you have time, could you see if this merges cleanly into release-3.6 after we merge it into main? Also happy to do it if you're busy.