test: Handle libc++ shared lib name on FreeBSD

(I didn't take a guess at the Linux names, as these tests are currently
skipped with the comment "No standard locations for libc++ on Linux.")


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@190307 91177308-0d34-0410-b5e6-96231b3b80d8