Use stub in test to avoid difference in setup, how network requests take place

packages/subproviders/package.json

@@ -69,6 +69,7 @@
         "@types/lodash": "4.14.104",
         "@types/mocha": "^2.2.42",
         "@types/node": "^8.0.53",
+        "@types/sinon": "^2.2.2",
         "chai": "^4.0.1",
         "chai-as-promised": "^7.1.0",
         "copyfiles": "^1.2.0",
@@ -78,6 +79,7 @@
         "npm-run-all": "^4.1.2",
         "nyc": "^11.0.1",
         "shx": "^0.2.2",
+        "sinon": "^4.0.0",
         "tslint": "5.8.0",
         "typedoc": "0xProject/typedoc",
         "typescript": "2.7.1",

packages/subproviders/test/unit/redundant_rpc_subprovider_test.ts

@@ -1,6 +1,7 @@
 import { DoneCallback } from '@0xproject/types';
 import * as chai from 'chai';
 import { JSONRPCResponsePayload } from 'ethereum-types';
+import * as Sinon from 'sinon';
 import Web3ProviderEngine = require('web3-provider-engine');
 import RpcSubprovider = require('web3-provider-engine/subproviders/rpc');
 
@@ -41,6 +42,9 @@ describe('RedundantSubprovider', () => {
         const nonExistentSubprovider = new RpcSubprovider({
             rpcUrl: 'http://does-not-exist:3000',
         });
+        const handleRequestStub = Sinon.stub(nonExistentSubprovider, 'handleRequest').throws(
+            new Error('REQUEST_FAILED'),
+        );
         const subproviders = [nonExistentSubprovider as Subprovider, ganacheSubprovider];
         const redundantSubprovider = new RedundantSubprovider(subproviders);
         provider.addProvider(redundantSubprovider);
@@ -55,6 +59,7 @@ describe('RedundantSubprovider', () => {
         const callback = reportCallbackErrors(done)((err: Error, response: JSONRPCResponsePayload) => {
             expect(err).to.be.a('null');
             expect(response.result.length).to.be.equal(DEFAULT_NUM_ACCOUNTS);
+            handleRequestStub.restore();
             done();
         });
         provider.sendAsync(payload, callback);