IntegrationTestMixinTransformation still references JUnit 4 RunWith

[longwa]

This transformation references the JUnit 4 class RunWith which has been replaced with @ExtendWith

grails-testing-support/grails-testing-support/src/main/groovy/org/grails/compiler/injection/testing/IntegrationTestMixinTransformation.groovy

Line 20 in bdf0619

import org.junit.runner.RunWith

This requires the JUnit 4 libraries to still be available in order to run integration tests in Grails 5.