docs: add documentation for `LIBCXX_INCLUDE_TESTS`

Add some missing documentation for the `LIBCXX_INCLUDE_TESTS` option.

Patch by Jean Heyd Meneide!

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@365154 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed