add test for is_event_assignment

[paulflang]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

There was a bug when parameters or compartments are not constant and modified in an event. This PR is a fix.