test: track simulated timers per dispatcher to simplify thread interactions and locking

[jmarantz]

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Commit Message:
Direct copy of @antoniovicente 's #12609 , with format fixed (one spelling dict add).
#Additional Description:
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[jmarantz]

After having merged in Matt's changes to de-flake integration tests, 3 fail locally for me in fastbuild with this PR:

//test/extensions/filters/http/fault:fault_filter_integration_test      TIMEOUT in 315.1s
//test/integration:http_timeout_integration_test                        TIMEOUT in 315.1s
//test/server:guarddog_impl_test                                         FAILED in 17.1s

One possible place to look is SimulatedTimeSystemHelper::SimulatedScheduler::runReadyAlarms and avd's comment:

  // TODO delay alarms scheduled this iteration until next iteration.

or it could just be test bugs we were getting away with before.

[jmarantz]

closing this for now in favor of Antonio's pr