fix(gnutls): replace UDP test skip with readiness polling

Wait for the UDP socket to bind before starting the client, detect
server exit, and clean up on skip. Preserve both handshake checks.

Add regression checks and document the rationale for upstream submission.

Validated on x86-64-v3: 796 passes, 131 existing skips, zero failures.
All seven focused checks and the delayed-start reproduction passed.

overlays/default.nix

@@ -15,6 +15,7 @@
     };
   };
 
+  patches = import ./patches;
   test-exclusions = import ./test-exclusions.nix;
   x86-64-v3-workarounds = import ./x86-64-v3-workarounds.nix;
 }