fix: make mocha tests working again

Author: wszydlak

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "third_party/protobuf"]
 	path = third_party/protobuf
 	url = https://github.com/protocolbuffers/protobuf.git
+[submodule "third_party/protobuf-javascript"]
+	path = third_party/protobuf-javascript
+	url = https://github.com/protocolbuffers/protobuf-javascript.git

packages/grpc-web/test/generated_code_test.js

@@ -473,7 +473,7 @@ describe('grpc-web generated code (closure+grpcwebtext)', function() {
     ".",
     "../../../javascript",
     "../node_modules/google-closure-library",
-    "../../../third_party/protobuf/js",
+    "../../../third_party/protobuf-javascript",
   ].map(jsPath => path.relative(cwd, path.resolve(__dirname, jsPath)));
   const closureArgs = [].concat(
     jsPaths.map(jsPath => `--js=${jsPath}`),