Summary:
As a step in the plan to migrate LogDevice to using fbcode_builder as
the primary build method, allow LogDevice to build without submodules.
In place of the self compiled modules, the CMake scripts will look in
the system (or provided prefix locations) and use precompiled modules
there. Users can then use fbcode_builder to ensure those required
modules are built and available.

* Use new 'folly_test_util' library rather than referencing sources
directly
Pull Request resolved: https://github.com/facebookincubator/LogDevice/pull/71

Reviewed By: AhmedSoliman

Differential Revision: D14578119

Pulled By: calebmarchent

fbshipit-source-id: 36400d41d3f1906919706d20d0fb53195fec2e63