Added a basic connection test for lldb-gdbserver.

The lldb-gdbserver tests are skipped if the lldb-gdbserver exe cannot be found (currently only built for Linux and FreeBSD).  The lldb-gdbserver exe is found by the LLDB_EXEC environment variable, using the same dir as the lldb exe, and then replacing lldb with lldb-gdbserver.

llvm-svn: 206731