Separate flakey from clowny tests

Summary: - add --flakey option to run them anyway
 - make it so you don't need to blow away framework_downloads/WHATEVER every time you remove something from the flakeylist/clownlist/blacklist or add --flakey

The idea is that we will still run flakey tests for the charts on hhvm.com/frameworks/ , but we won't include them in our commit-blocking test runs.

Reviewed By: @JoelMarcey

Differential Revision: D1398512