[test][lldb-vscode] Fix unclean shutdown in test_terminate_commands

This test manually sends `terminateCommands` to shutdown, but then the test attempts another `terminateCommands` request in test tear down, which fails since it's already torn down. This error is ignored in LLDB's old fork of unittest2, but will be reported as a test failure in `unittest` from the Python standard library. Use `disconnectAutomatically` to avoid terminating twice.