[testing] dump the cache when depending on warning messages

It's possible to configure Jest to not dump the module cache between
specs. This makes it tricky when we silence warnings one a 2nd call.

In this case, the same message was getting logged so when we expected
the count of warning calls to increment, it didn't.

src/browser/ui/dom/components/__tests__/ReactDOMInput-test.js

@@ -22,6 +22,7 @@ describe('ReactDOMInput', function() {
   var ReactTestUtils;
 
   beforeEach(function() {
+    require('mock-modules').dumpCache();
     React = require('React');
     ReactLink = require('ReactLink');
     ReactTestUtils = require('ReactTestUtils');