Include testlib files in JSI

Summary: This adds the testlib.cpp/h files to external JSI. They're in a `test/` subdirectory so that build scripts using globs like `*.cpp` won't include them.

Reviewed By: mhorowitz

Differential Revision: D15582281

fbshipit-source-id: 1785ee5071fcf98e92fbf3a11eddb21fe84b3799

Author: willholen