[CMake] Run the lldb-server tests with system debugserver.
[CMake] Run the lldb-server tests with system debugserver.

Now that we no longer build debugserver when LLDB_USE_SYSTEM_DEBUGSERVER
is set, we have to change the logic for testing lldb-server.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@372900 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 464120024a5eb9565faf30d939a479d12d53d5c0)

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb