Make tests opt-in via -DTESTS=ON (off by default)

Summary:
- Add a `TESTS` CMake option (default `OFF`) that guards `find_package(GTest)`, `include(GoogleTest)`, the `test_main` library, `rebalancer_tests.exe`, `enable_testing()`, `gtest_discover_tests()`, and the Python-based macro/path check tests
- Mirrors the existing `EXAMPLES` and `BENCHMARKS` options — pass `-DTESTS=ON` to build tests
- Add `TESTS=ON` to `[cmake.defines.test=on]` in the getdeps manifest so CI (which runs `getdeps build` without `--no-tests`) continues to build and run tests automatically; SDK and wheel builds already pass `--no-tests` and are unaffected

X-link: https://github.com/facebookincubator/rebalancer/pull/22

Reviewed By: kvelakur

Differential Revision: D109102488

Pulled By: r-barnes

fbshipit-source-id: 77679978a42988fc1311aa6753ceea18e00d4cde

Author: r-barnes

build/fbcode_builder/manifests/rebalancer

@@ -25,6 +25,7 @@ CMAKE_POSITION_INDEPENDENT_CODE=ON
 # workflow's --extra-cmake-defines.
 [cmake.defines.test=on]
 EXAMPLES=ON
+TESTS=ON
 
 [dependencies]
 boost