e2e: fix race in TestFailingReplication

[timvaillancourt]

Description

This PR fixes a race condition in the vtbackup e2e test that causes TestFailingReplication (and other tests using prepareCluster()) to hang in CI.

The problem

prepareCluster() calls restore(), which returns as soon as the vttablet HTTP endpoint responds. But RestoreData continues running in the background holding the actionSema — a weight-1 semaphore. The next call, SetWritable, also needs the actionSema, so it blocks waiting for the restore to finish.

This race has always existed on all branches, but became a consistent failure on main and release-23.0 because MySQL 8.4 restores take longer than MySQL 8.0, widening the race window enough that SetWritable fires before RestoreData completes. In CI this causes a 19+ minute hang followed by a timeout.

The fix

Adds a waitForRestoreComplete() helper between restore() and SetWritable that polls the tablet type via /debug/vars, waiting for the replica → restore → replica transition that signals RestoreData has finished and released the semaphore.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

AI Disclosure

Fix found by Claude while debugging flakiness on PR #19472

[vitess-bot]

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