[lldb] Fix TestPlatformProcessConnect.py

The test was marked as remote-only, which means it was run ~never, and
accumulated various problems. This commit modifies the test to run
locally and includes a couple of other fixes necessary to make it run:
- moves the "invoke" method into the "Base" test class
- adds []'s around the IP address in a couple more places to make things
  work with IPv6

The test is now marked as skipped when running the remote test suite. It
would be possible to make it run both locally and remotely, but this
would require writing a lot special logic for the remote case, and that
is not worth it.