Fix rewatch lock races in watcher tests

[cknitt]

Summary

• Make rewatch lock acquisition atomic so concurrent processes cannot both acquire the same lockfile.
• Update watcher test helpers to wait for spawned watcher processes to exit after removing watch.lock.
• Adapt the lock/watch tests so sequential test runs do not leave overlapping watchers in the same fixture workspace.

Why

CI was intermittently failing with corrupted compiled interface artifacts while running rewatch tests. The root causes were lock/test races:

• lock acquisition used a check-then-create flow, allowing concurrent processes to both believe they acquired the same lock;
• the shell test helper removed watch.lock but did not wait for the watcher process to terminate before the next test started.

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commit: b68a935

[jfrolich]

Hmm why did this just remove the lockfile, that should be removed when killed

[jfrolich]

Ah deleting the lock file would kill the watcher. Is that not the case anymore. Why do we need this complex setup. The pid is also in the lockfile, so we shouldn't need to pass it. But if it behaves correctly it should also exit so we shouldn't need to kill it. Maybe we need to await that it's really closed though? not sure though. At least killing shouldn't be needed, that's exactly what this test is testing. If it fails the behavior is wrong.

[cknitt]

@jfrolich Yeah there may have been some hallucinations here, sorry! Please have another look.
Now just waiting for the pid to disappear after removing the watcher lock file.