mvcc: Deflake TestWatchVictims with synctime

[Hrishi-Baskaran]

Resolves #22266

Uses the synctest package to remove nondeterminism from the TestWatchVictims. The main issue with TestWatchVictims was the use of a wall clock 10 second timeout on Watch operations as a naive deadlock detection mechanism. On slow CI machines this may not be enough, on fast CI machines this may be too long, slowing down the test. I was able to replicate this with stress

    watchable_store_test.go:944: time out

(see log with more context in the issue)

This PR replaced the above mechanism with a more deterministic one by running relevant code in a synctest bubble which can halt or advance time arbitrarily depending on the current state of the synctest bubble.

The end result is a test that will quickly detect a deadlock on a fast machine but will not report a deadlock if goroutines are advancing, even on a slow machine.

This also helps enforce the goal of the test that Puts should happen faster than Watch events by back pressuring Watch events with simulated time. This was previously done with wall clock time which is not as reliable and can slow down fast CI machines.

The overall changes improve both determinism and performance of this test.

[kubernetes-prow]

Hi @Hrishi-Baskaran. Thanks for your PR.

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[kubernetes-prow]

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This pull-request has been approved by: Hrishi-Baskaran
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[Hrishi-Baskaran]

/assign @serathius