[tests] Enable use_system_lib support on Linux.
 - Patch by Michael van der Westhuizen.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174527 91177308-0d34-0410-b5e6-96231b3b80d8
[tests] Change test default to run against locally built library.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174528 91177308-0d34-0410-b5e6-96231b3b80d8