Cleanup threads for CacheBuilderTest and FuturesTest

[aoli-al]

This PR addresses #7319. Cleanup threads for CacheBuilderTest and FuturesTest

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[cpovirk]

(Thanks, I expect to be interested in merging this if you can get the CLA sorted out!)

[aoli-al]

Thanks! It seems that the CLA is cleared https://github.com/google/guava/pull/7320/checks?check_run_id=27685732789

[ben-manes]

fwiw, technically these should shutdown if the test fails. Typically a try-finally would be ideal, but if you want to avoid that then overriding setUp and tearDown could work by creating a Closer per test instance and registering the shutdown with that.

[aoli-al]

Will it be okay to make service a class field and then initialize it in each test. tearDown should call service.shutdown if service is not null?

[cpovirk]

I think the most important thing is shutting down when the test succeeds so that there aren't threads hanging around during some later test run that someone tries to attach a debugger to. [edit: Or, of course, it would be bad if we had so many threads that that actually causes resource problems.] And I think we're inconsistent about whether we bother to try to shut down in case of failure.

Let me see whether the internal reviewer is concerned or not, hopefully tomorrow.