run-test: pass --experimental-test-module-mocks through to node

Specs that replace a module need mock.module from node:test, which
Node only exposes behind this flag. The runner rejected unknown flags,
so there was no way to turn it on. It is now accepted and forwarded to
node like --import and --require.

The repo's own test run enables it, and a spec checks that a module
registered as a mock is what a later dynamic import receives.

packages/dev/scripts/pezkuwi-dev-run-test.mjs

@@ -58,6 +58,11 @@ for (let i = 0; i < args.length; i++) {
       cmd.push(args[++i]);
       break;
 
+    // node flags with no params
+    case '--experimental-test-module-mocks':
+      nodeFlags.push(args[i]);
+      break;
+
     // node flags that could have additional params
     case '--import':
     case '--loader':