tests: add reproducer for reconnect gating behind disconnect notify

[joejulian]

Summary

Add a failing regression-style reproducer for the case where reconnect progress is gated behind synchronous RPC_CLNT_DISCONNECT notify handling.

What this does

• adds a test-only client option to block disconnect notify on a holdfile
• adds a test that:
  • mounts a simple volume
  • forces a brick disconnect
  • verifies the disconnect-notify hold hook fired
  • restarts the brick
  • expects a second successful client attach while notify is still blocked

Current result

The test fails reliably with the patched /usr/local Gluster test stack:

• the hold hook logs
• the brick comes back
• a second Connected, attached to remote volume never appears while notify is blocked

That demonstrates reconnect completion is gated by synchronous disconnect-notify handling.

Notes

This reproduces the reconnect-path bug, not the original cause of the initial FUSE stall.