Fix tests related to event-target-shim

Summary: @​public

Jest has an issue where if you export `Symbol` it crashes badly. Since event-target-shim does that, it breaks everything. Mocking that module fixes the issue until @cpojer comes back from vacation and fixes the bug in Jest itself.

Reviewed By: @jingc

Differential Revision: D2503562

Libraries/WebSocket/__mocks__/event-target-shim.js

@@ -0,0 +1,9 @@
+// Jest fatals for the following statement (minimal repro case)
+//
+//   exports.something = Symbol;
+//
+// Until it is fixed, mocking the entire node module makes the
+// problem go away.
+
+'use strict';
+module.exports = function() {};