test: Attempt to re-enable iam#testPermissions (#429)

Author: frankyn

system-test/storage.ts

@@ -1503,13 +1503,10 @@ describe('storage', () => {
              });
            }));
 
-        // @TODO: There may be a backend bug here.
-        // Reference:
-        // https://github.com/googleapis/nodejs-storage/pull/190#issuecomment-388475406
-        it.skip('iam#testPermissions', doubleTest((options, done) => {
-                  const tests = ['storage.buckets.delete'];
-                  bucketNonWhitelist.iam.testPermissions(tests, options, done);
-                }));
+        it('iam#testPermissions', doubleTest((options, done) => {
+             const tests = ['storage.buckets.delete'];
+             bucketNonWhitelist.iam.testPermissions(tests, options, done);
+           }));
 
         it('notification#get', doubleTest((options, done) => {
              if (!notification) {