Grails 3: Unit, integration and functional tests compiles to build/classes/test/ which causes problems when testing package

All tests (unit, integration and functional) from a Grails 3 project all compiles to `build/classes/test/` from Idea, but in Grails they compile to separate folders in `build/classes`.
This causes problems when running all tests in a package, because the runner mixes unit and integration tests.
Instead, integration/functional tests should compile to `build/classes/integration-test`

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*Imported from [IDEA-151499](https://youtrack.jetbrains.com/issue/IDEA-151499) · Type: Bug · Votes: 5*
*Comments, attachments, dates and reporter are not part of the export — follow the link above for the full history.*

Start by tracing how Grails 3 tests are compiled into build/classes/test/ from IntelliJ IDEA, comparing that with Grails' separate build/classes locations. Verify unit, integration, and functional test outputs, then confirm that integration and functional classes are separated so package test runs no longer mix them.

Written by the indexing model from the issue text.