[autotools->cmake] Move add_subdirectory(test) inside CLANG_INCLUDE_TESTS to match the behavior of the LLVM where LLVM_INCLUDE_TESTS controls whether tests is included.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188760 91177308-0d34-0410-b5e6-96231b3b80d8