Summary:
in getdeps we currently don't build and run the tests
There are a few issues:
  1. we need to also build tests for fizz, wangle, mvfst since proxygen tests include headers only exported if building tests in dependencies
  2. we use `ExternalProject_add` for gtest/gmock. but doesn't seem to be playing nicely with getdeps

Reviewed By: dddmello, mjoras

Differential Revision: D16934955

fbshipit-source-id: fb1c52237f9f0c71da86643409972c94d16e6a71