Add defensive isMounted checks before setting state via useMutation Hook

[alessbell]

This PR adds checks in two places inside of useMutation to verify that the component is mounted before setting state with setResult, which fixes #9858.

Testing

Spying on console.error here is sufficient. I've updated the tests, which now fail if either isMounted check added in the PR is removed.

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

[dylanwulf]

It looks like I'm at an impasse when it comes to making assertions against component state after the component unmounts (I want to call unmount in the middle of my test—it's also called automatically after each test via cleanup function—in order to ensure the state hasn't changed), as subsequent calls to waitForNextUpdate time out. Would love any suggestions

What I did in #9801 was just spy on console.error and assert that it never got called. Probably not the best solution, but it works!

[alessbell]

What I did in #9801 was just spy on console.error and assert that it never got called. Probably not the best solution, but it works!

For this case, that should suffice :) Updated - thanks!

[alessbell]

@benjamn removing either isMounted check this PR adds now fails the test (thanks to @dylanwulf's suggestion 👍). Updated and squashed into a single commit.

[hwillson]

Woohoo - this is really great @alessbell (and thanks for the spying on console.error suggestion @dylanwulf 🙇). A big 👍 from me!