Error due to Drupal 9 support

In what version of drupal is this an issue? 8.7 or 8.8?

And what error do you get and how does it manifest itself?

Do you have $settings['extension_discovery_scan_tests'] = TRUE; set in settings.php by any chance?

Depending on if that is set true, then #3096609: Allow contrib test modules to not need a core or core_version_requirement key is probably what is causing your issues.

I'm getting the same error after moving to core 8.8.1

In InfoParserDynamic.php line 68:

  The 'core' or the 'core_version_requirement' key must be present in modules
  /contrib/migrate_plus/tests/modules/migrate_plus_test/migrate_plus_test.inf
  o.yml

I do not have $settings['extension_discovery_scan_tests'] = TRUE; set and still get the error after explicitly setting it to false, although a a scan of my code base does not show it set anywhere else.

Using the patch got me back to work, but I wonder if there is an error in the new core code since this occurred right after upgrading to 8.8.1

#3096609: Allow contrib test modules to not need a core or core_version_requirement key is only in 8.8.x it's not made it into a release yet :(

Co-incidental then - I must have also upgraded migrate_plus around the same time

Committed and pushed.