add tests for test-resolver with alternative resolver

packages/grpc-js/gulpfile.ts

@@ -81,6 +81,21 @@ const runTests = checkTask(() => {
   );
 });
 
-const test = gulp.series(install, copyTestFixtures, runTests);
+const testWithAlternativeResolver = checkTask(() => {
+  process.env.GRPC_NODE_USE_ALTERNATIVE_RESOLVER = 'true';
+  return gulp.src(`${outDir}/test/**/test-resolver.js`).pipe(
+    mocha({
+      reporter: 'mocha-jenkins-reporter',
+      require: ['ts-node/register'],
+    })
+  );
+});
+
+const test = gulp.series(
+  install,
+  copyTestFixtures,
+  runTests,
+  testWithAlternativeResolver
+);
 
 export { install, lint, clean, cleanAll, compile, test };

packages/grpc-js/test/test-resolver.ts

@@ -460,7 +460,7 @@ describe('Name Resolver', () => {
             }
           },
         },
-        {}
+        { 'grpc.dns_min_time_between_resolutions_ms': 2000 }
       );
       resolver.updateResolution();
       setTimeout(() => {