Fix thread starvation in test_single_channel_multiple_mpp

The busy-wait loop polling for PaymentClaimed events had no yield,
causing it to continuously acquire ChannelManager locks via
get_and_clear_pending_events(). This could starve the claim_funds
thread of lock access, preventing it from ever queuing the event.
Add a yield_now() call matching the pattern used by the other two
spin loops in this test.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jkczyz

lightning/src/ln/chanmon_update_fail_tests.rs

@@ -4715,6 +4715,9 @@ fn test_single_channel_multiple_mpp() {
 				}
 				have_event = true;
 			}
+			if !have_event {
+				std::thread::yield_now();
+			}
 		}
 	});