Replace Fabric JUnit hacks with fabric-loader-junit

Also configure some of our common JUnit run configurations via Gradle -
I end up setting these up in every worktree anyway - so let's just do it
once.

buildSrc/src/main/kotlin/cc/tweaked/gradle/CCTweakedPlugin.kt

@@ -9,6 +9,10 @@ import org.gradle.api.Project
 import org.gradle.api.plugins.JavaPlugin
 import org.gradle.api.plugins.JavaPluginExtension
 import org.gradle.jvm.toolchain.JavaLanguageVersion
+import org.gradle.plugins.ide.idea.model.IdeaModel
+import org.jetbrains.gradle.ext.IdeaExtPlugin
+import org.jetbrains.gradle.ext.runConfigurations
+import org.jetbrains.gradle.ext.settings
 
 /**
  * Configures projects to match a shared configuration.
@@ -21,6 +25,20 @@ class CCTweakedPlugin : Plugin<Project> {
             val sourceSets = project.extensions.getByType(JavaPluginExtension::class.java).sourceSets
             cct.sourceDirectories.add(SourceSetReference.internal(sourceSets.getByName("main")))
         }
+
+        project.plugins.withType(IdeaExtPlugin::class.java) { extendIdea(project) }
+    }
+
+    /**
+     * Extend the [IdeaExtPlugin] plugin's `runConfiguration` container to also support [JUnitExt].
+     */
+    private fun extendIdea(project: Project) {
+        val ideaModel = project.extensions.findByName("idea") as IdeaModel? ?: return
+        val ideaProject = ideaModel.project ?: return
+
+        ideaProject.settings.runConfigurations {
+            registerFactory(JUnitExt::class.java) { name -> project.objects.newInstance(JUnitExt::class.java, name) }
+        }
     }
 
     companion object {

buildSrc/src/main/kotlin/cc/tweaked/gradle/IdeaExt.kt

@@ -0,0 +1,23 @@
+// SPDX-FileCopyrightText: 2023 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package cc.tweaked.gradle
+
+import org.jetbrains.gradle.ext.JUnit
+import javax.inject.Inject
+
+/**
+ * A version of [JUnit] with a functional [className].
+ *
+ * See [#92](https://github.com/JetBrains/gradle-idea-ext-plugin/issues/92).
+ */
+open class JUnitExt @Inject constructor(nameParam: String) : JUnit(nameParam) {
+    override fun toMap(): MutableMap<String, *> {
+        val map = HashMap(super.toMap())
+        // Should be "class" instead of "className".
+        // See https://github.com/JetBrains/intellij-community/blob/9ba394021dc73a3926f13d6d6cdf434f9ee7046d/plugins/junit/src/com/intellij/execution/junit/JUnitRunConfigurationImporter.kt#L39
+        map["class"] = className
+        return map
+    }
+}