CMake: Only add test/ subdirectory when it exists

r296685 started adding the test/ subdirectory even when
LIBCXX_INCLUDE_TESTS=OFF.  This is great for testing libcxx standalone,
but it also breaks the build when the test/ subdirectory is removed
(and our submission system strips all test/ directories).

This patch updates the logic to check for test/ before adding it.

rdar://problem/31931366

llvm-svn: 302095