Make it possible to compile openr through CMakeLists.txt
Make it possible to compile openr through CMakeLists.txt

Summary:
This diff fixes `CMakeLists.txt` to enable building `openr` tests using CMake:
1. It adds `add_openr_test` CMake function that adds executable target, registers it as test, links it with bunch of libraries like GTest and GMock, etc...
2. There is no `openr/tests/OpenrModuleTestBase.cpp` anywhere in the source tree, so this commit replaces it with `openr/common/Flags.cpp`.

Reviewed By: jstrizich

Differential Revision: D18584028

fbshipit-source-id: 07d854ef98d0d2509889a08ad042a371101a2825

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