[launch] improve test coverage for non-core classes

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`