fix issues with the open source tests

fix issues with the open source tests

Summary:
Fix build problems in the open source Makefiles that cause issues when running
`make check`

- The test subdirectory needs to be processed before experimental, since some
  tests in experimental/ depend on libfollytestmain from test/
- The stats/test Makefile had an incorrectly copy-and-pasted line for
  libgtest_la_SOURCES but didn't actually build a libgtest.la library.
- The test/ Makefile defined thread_id_test but forgot to add it to TESTS so
  that it would actually be run as part of "make check"

Reviewed By: Orvid

Differential Revision: D5249132

fbshipit-source-id: 5a71e1f72a39d5407b843a5876891c67238ec006