Remove IntelliJ JUnit 4 compatibility layer

[AB-xdev]

While performing #47 we encountered:
https://plugins.jetbrains.com/docs/intellij/tools-intellij-platform-gradle-plugin-faq.html#junit5-test-framework-refers-to-junit4
https://youtrack.jetbrains.com/issue/IJPL-159134

This is currently worked around by a compatibility layer that was chosen in favor of importing the JUnit4 library as this usually leads to incorrect imports.

https://youtrack.jetbrains.com/issue/IJPL-159134 was fixed in IDEA 2024.3+ (unreleased as of Sept 2024).
Once the minimal plugin version is >= 2024.3 this can be rechecked and eventually removed.

[AB-xdev]

Nothing was changed inside IntelliJ classes in 2024.3 EAP, however the issue was closed as "Fixed".
Problem is still present.

Let's wait some time maybe the change is still stuck somewhere...

[AB-xdev]

Apparently this was the "fix".
I see no code that fixes anything in there...

JUnit5TestSessionListener still references a JUnit4 TestCase (UsefulTestCase).

[AB-xdev]

This was maybe fixed with https://github.com/JetBrains/intellij-platform-gradle-plugin/releases/tag/2.2.0

• Make IntelliJ Platform dependencies available to testImplementation by default to avoid mutating tests classpath
• For running testing, use test sandbox instead of compiled classes from build/classes directory

[AB-xdev]

This was maybe fixed with https://github.com/JetBrains/intellij-platform-gradle-plugin/releases/tag/2.2.0

• Make IntelliJ Platform dependencies available to testImplementation by default to avoid mutating tests classpath
• For running testing, use test sandbox instead of compiled classes from build/classes directory

Nope, still not working

org.gradle.api.internal.tasks.testing.TestSuiteExecutionException: Could not start Gradle Test Executor 1.
	at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.startProcessing(SuiteTestClassProcessor.java:45)
	at java.base@21.0.5/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
	at java.base@21.0.5/java.lang.reflect.Method.invoke(Method.java:580)
	at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:36)
	at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:24)
	at org.gradle.internal.dispatch.ContextClassLoaderDispatch.dispatch(ContextClassLoaderDispatch.java:33)
	at org.gradle.internal.dispatch.ProxyDispatchAdapter$DispatchingInvocationHandler.invoke(ProxyDispatchAdapter.java:92)
	at jdk.proxy1/jdk.proxy1.$Proxy4.startProcessing(Unknown Source)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker$1.run(TestWorker.java:168)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.executeAndMaintainThreadName(TestWorker.java:132)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:103)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:63)
	at org.gradle.process.internal.worker.child.ActionExecutionWorker.execute(ActionExecutionWorker.java:56)
	at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:121)
	at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:71)
	at worker.org.gradle.process.internal.worker.GradleWorkerMain.run(GradleWorkerMain.java:69)
	at worker.org.gradle.process.internal.worker.GradleWorkerMain.main(GradleWorkerMain.java:74)
Caused by: java.util.ServiceConfigurationError: org.junit.platform.launcher.LauncherSessionListener: Provider com.intellij.tests.JUnit5TestSessionListener could not be instantiated
	at java.base/java.util.ServiceLoader.fail(ServiceLoader.java:586)
	at java.base/java.util.ServiceLoader$ProviderImpl.newInstance(ServiceLoader.java:813)
	at java.base/java.util.ServiceLoader$ProviderImpl.get(ServiceLoader.java:729)
	at java.base/java.util.ServiceLoader$3.next(ServiceLoader.java:1403)
	at java.base/java.lang.Iterable.forEach(Iterable.java:74)
	at org.junit.platform.launcher.core.LauncherFactory.createLauncherSessionListener(LauncherFactory.java:169)
	at org.junit.platform.launcher.core.LauncherFactory.lambda$openSession$0(LauncherFactory.java:101)
	at org.junit.platform.launcher.core.DefaultLauncherSession.<init>(DefaultLauncherSession.java:52)
	at org.junit.platform.launcher.core.LauncherFactory.openSession(LauncherFactory.java:100)
	at org.junit.platform.launcher.core.LauncherFactory.openSession(LauncherFactory.java:82)
	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor$BackwardsCompatibleLauncherSession.open(JUnitPlatformTestClassProcessor.java:323)
	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor.createTestExecutor(JUnitPlatformTestClassProcessor.java:86)
	at org.gradle.api.internal.tasks.testing.junit.AbstractJUnitTestClassProcessor.startProcessing(AbstractJUnitTestClassProcessor.java:44)
	at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.startProcessing(SuiteTestClassProcessor.java:43)
	... 16 more
Caused by: java.lang.NoClassDefFoundError: junit/framework/TestCase
...