Fix jest mocking in tests (#592)

Summary:
Pull Request resolved: https://github.com/facebook/flipper/pull/592

Finally we can join the 21st century and use `jest.mock` in tests!

I'm enabling the ts-jest transform to `*/__tests__/*.tsx` files. This takes care of transforming the typescript, and preserves jest mock capabilities.
I have left the existing custom transform on all other files, so they will continue to work as usual.

Caveats:
* Our existing custom transforms will no longer be applied in tests.
  * With our current suite this makes no difference. But bear in mind things like importing directly from 'flipper' instead of the actual files, won't work. However, I don't see this being a problem, as I think they'll only be needed inside the actual product code.

Reviewed By: passy

Differential Revision: D17978753

fbshipit-source-id: df7b312b5dd5592e4aa4f8df605f5952ea3158e9

src/fb-stubs/__mocks__/Logger.tsx

@@ -0,0 +1,51 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import {Store} from '../../reducers/index';
+import {getStringFromErrorLike} from '../../utils/errors';
+import {Args, Logger, TrackType} from '../../fb-interfaces/Logger';
+
+export function extractError(
+  ...data: Array<any>
+): {message: string; error: Error} {
+  const message = data.map(getStringFromErrorLike).join(' ');
+  const error = data.find(e => e instanceof Error) || new Error(message);
+  return {
+    message,
+    error,
+  };
+}
+
+export class FBLogger implements Logger {
+  constructor(_store?: Store, _args?: Args) {}
+
+  track(_type: TrackType, _event: string, _data?: any, _plugin?: string) {}
+
+  trackTimeSince(_mark: string, _eventName?: string) {}
+
+  info = (..._data: Array<any>) => {};
+
+  warn = (..._data: Array<any>) => {};
+
+  error = (..._data: Array<any>) => {};
+
+  debug = (..._data: Array<any>) => {};
+
+  getLogs(): Array<string> {
+    return [];
+  }
+}
+
+export function init(_store: Store, _args?: Args): Logger {
+  return new FBLogger();
+}
+
+export function getInstance(): Logger {
+  return new FBLogger();
+}