getdeps: update run_cmake.py to also support running tests
getdeps: update run_cmake.py to also support running tests

Summary: Add a `--mode=tests` behavior for invoking `ctest`

Reviewed By: simpkins

Differential Revision: D17610033

fbshipit-source-id: 6284fe1dbfb7adf38a67820c842a5531eefd72fa

facebook/wangle,facebook/wangle,facebook/wangle