Moved `lit` test that runs unit tests for benchmark scripts from `validation-test/Python` to `test/benchmark` directory. Run the tests for benchmark infrustructure with single lit invocation:
````
swift-source$ ./llvm/utils/lit/lit.py -sv ${SWIFT_BUILD_DIR}/test-macosx-x86_64/benchmark
````
Documented the invocation of benchmark infrastructure tests in README.md
