* Fix volume repeatedly toggling between crowded and uncrowded
Fixes#6712
The issue was that removeFromCrowded() was called in removeFromWritable(),
which is invoked whenever a volume temporarily becomes unwritable (due to
replica count fluctuations, heartbeat issues, or read-only state changes).
This caused unnecessary toggling:
1. Volume becomes temporarily unwritable → removeFromWritable() →
removeFromCrowded() logs 'becomes uncrowded'
2. Volume becomes writable again
3. CollectDeadNodeAndFullVolumes() runs → setVolumeCrowded() logs
'becomes crowded'
The fix:
- Remove removeFromCrowded() call from removeFromWritable()
- Only clear crowded status when volume is fully unregistered from
the layout (when location.Length() == 0 in UnRegisterVolume)
This ensures transient state changes don't cause log spam and the
crowded status accurately reflects the volume's size relative to
the grow threshold.
* Refactor test to use subtests for better readability
Address review feedback: use t.Run subtests to make the test's intent
clearer by giving each verification step a descriptive name.
