[launch] improve test coverage for non-core classes

[wjwwood]

Currently there are minimal tests for some of the core classes like LaunchDescription, LaunchService, and Action, but there's not really any tests for things like:

• launch.LaunchInstrospector
• launch.Substitution
• anything in the namespaces:
  • launch.actions
  • launch.event_handlers
  • launch.events
  • launch.substitutions
  • launch.utilities

[jacobperron]

Tests added:

• [ ] launch.LaunchInstrospector
• [ ] launch.Substitution
• [ ] launch.actions
  • [x] declare_launch_argument.py
  • [ ] emit_event.py
  • [ ] execute_process.py
  • [x] group_action.py
  • [x] include_launch_description.py
  • [ ] log_info.py
  • [ ] opaque_function.py
  • [x] pop_launch_configurations.py
  • [x] push_launch_configurations.py
  • [x] register_event_handler.py
  • [x] set_launch_configuration.py
  • [ ] timer_action.py
  • [x] unregister_event_handler.py
  • [x] unset_launch_configuration.py
• [ ] launch.event_handlers
  • [ ] event_named.py
  • [x] on_include_launch_description.py (#177)
  • [x] on_process_exit.py (#177)
  • [x] on_process_io.py (#177)
  • [x] on_shutdown.py (#177)
• [ ] launch.events
  • [ ] include_launch_description.py
  • [ ] process
    • [ ] process_exited.py
    • [ ] process_io.py
    • [ ] process_matchers.py
    • [ ] process_started.py
    • [ ] process_stderr.py
    • [ ] process_stdin.py
    • [ ] process_stdout.py
    • [ ] process_targeted_event.py
    • [ ] running_process_event.py
    • [ ] shutdown_process.py
    • [ ] signal_process.py
  • [ ] shutdown.py
  • [ ] timer_event.py
• [ ] launch.substitutions
  • [ ] environment_variable.py
  • [ ] local_substitution.py
  • [ ] python_expression.py
  • [ ] text_substitution.py
  • [ ] find_executable.py
  • [ ] launch_configuration.py
  • [ ] substitution_failure.py
  • [x] this_launch_file_dir.py
• [x] launch.utilities (#143)
  • [x] class_tools_impl (#142)
  • [ ] create_future_impl
  • [ ] normalize_to_list_of_substitutions_impl
  • [ ] signal_management
  • [ ] ensure_argument_type_impl
  • [ ] perform_substitutions_impl
  • [ ] visit_all_entities_and_collect_futures_impl