Retry flash dismissal until it lands, instead of clicking once

[sethherr]

dismiss_flash_messages clicked each close button once, then waited 10s for the region to clear. But ui--alert wires its close action in connect and application.js lazy loads controllers, so a click landing before that module arrives hits a button with no listener and is silently swallowed — and every caller dismisses right after a page load, which is exactly when that's most likely. Under CPU load it failed 1 run in 8.

• Click again until the region clears, the way open_modal already handles the same lazy-load race a few lines up. Same ~10s ceiling; under the load that reproduced the failure, 16 runs (80 dismissals) came back clean.
• Presence is asserted up front instead of by minimum: 1 inside the loop, so a retry can't raise a fresh ExpectationNotMet when the region clears between the probe and the lookup. retry_on_detach wraps the query rather than one click, so it re-finds instead of re-clicking a node that's already gone.

[sethherr]

Solving with #4031