Default Grails Integration tests to run test-app (and not JUnit)

For Grails applications, one should not have to select whether they wish to run an integration test as a Grails or JUnit test, but should instead be defaulted to Grails - as most integration tests - if designed properly - will not run using JUnit. This can be a source of unnecessary confusion.

I could see the argument for allowing this default to be overrided, but from a convention over configuration perspective, this seems to be a sensible default.

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*Imported from [IDEA-64167](https://youtrack.jetbrains.com/issue/IDEA-64167) · Type: Bug · Votes: 1*
*Comments, attachments, dates and reporter are not part of the export — follow the link above for the full history.*

Start by locating the Grails integration-test run configuration and the code that chooses between test-app and JUnit. Reproduce the current selection behavior with a Grails integration test, then verify that test-app is the default while the requested behavior remains testable.

Written by the indexing model from the issue text.