fix(overlays): stabilize timing-sensitive tests

overlays/test-exclusions.nix

@@ -4,10 +4,10 @@
 # nixpkgs binary cache cannot be used and upstream test suites run locally.
 # The jeeves builder uses /tmp/nix-builds so filesystem tests run on tmpfs
 # instead of ZFS with normalization=formD and utf8only=on; those tests remain
-# enabled. The remaining workarounds cover UDP readiness and nested-worker
-# scheduling races, plus architecture-dependent floating-point differences
-# whose risk we accept for our workloads. Keep these exceptions visible until
-# their causes are fixed.
+# enabled. The remaining workarounds cover UDP readiness, nested-worker
+# scheduling races, and mismatched timeout clocks, plus architecture-dependent
+# floating-point differences whose risk we accept for our workloads. Keep these
+# exceptions visible until their causes are fixed.
 _final: prev: {
   gnutls = prev.gnutls.overrideAttrs (old: {
     # This test uses a fixed four-second sleep instead of checking UDP
@@ -19,10 +19,23 @@ _final: prev: {
 
   pythonPackagesExtensions = prev.pythonPackagesExtensions ++ [
     (_pythonFinal: pythonPrev: {
+      backrefs = pythonPrev.backrefs.overridePythonAttrs (old: {
+        # regex measures its timeout in process CPU time, while this test used
+        # wall time and could miss the timeout when a busy builder descheduled it.
+        postPatch = (old.postPatch or "") + ''
+          substituteInPlace tests/test_bregex.py \
+            --replace-fail "time.time()" "time.process_time()"
+        '';
+      });
+
       pytest-xdist = pythonPrev.pytest-xdist.overridePythonAttrs (old: {
-        # Its acceptance tests start nested worker pools. Limit the outer pool
-        # so deliberately crashing inner workers do not race under heavy load.
-        preCheck = builtins.replaceStrings [ "--numprocesses=$NIX_BUILD_CORES" ] [ "--numprocesses=8" ] (
+        # The suite exercises its own worker pools. Run the outer suite with one
+        # worker and allow inner workers more time on heavily loaded builders.
+        postPatch = (old.postPatch or "") + ''
+          substituteInPlace testing/test_remote.py \
+            --replace-fail "WAIT_TIMEOUT = 10.0" "WAIT_TIMEOUT = 60.0"
+        '';
+        preCheck = builtins.replaceStrings [ "--numprocesses=$NIX_BUILD_CORES" ] [ "--numprocesses=1" ] (
           old.preCheck or ""
         );
       });