fix(overlays): exclude failing x86-64-v3 tests

Skip the flaky GnuTLS UDP readiness test and the SciPy FFT tests whose
precision differences are acceptable for our workloads.

overlays/default.nix

@@ -15,6 +15,8 @@
     };
   };
 
+  test-exclusions = import ./test-exclusions.nix;
+
   python-env = final: _prev: {
     my_python = final.python314.withPackages (
       ps: with ps; [