[tests] Add a 'use_system_lib' parameter.

 - This controls whether to execute against the locally built library or
   not. The default is currently True which maps to what was already being done
   by default.

 - I'd appreciate it if someone can implement the proper handling of this flag
   on linux, I no longer remember the details of its .so handling.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174404 91177308-0d34-0410-b5e6-96231b3b80d8