instrumentation tests are executed with BUCK

Summary:Added ability to run instrumentation tests with BUCK.

This change uses BUCK to build and run instrumentation tests facebook style.
The gains are that we can execute the same tests internally at FB and in OSS.
Also running tests not via graddle:connect command is 1.5 minutes faster.

I'll keep keep an eye on stability Gradle and BUCK builds for a while.
Closes https://github.com/facebook/react-native/pull/6176

Differential Revision: D2999878

Pulled By: bestander

fb-gh-sync-id: d715ba231769e57100685a1256f2e530c589921c
shipit-source-id: d715ba231769e57100685a1256f2e530c589921c