Share test fixtures across modules

- Move art.arcane.iris.testsupport into core/src/testFixtures and apply java-test-fixtures to core
- Consume core test fixtures from the bukkit plugin, nms, probe and the three loader adapters
- Delete the duplicate ProjectPaths copies in the bukkit-plugin and modded-common test trees
- Point ModdedMessageContributorTest at the shared ProjectPaths
- Guard CommandPackCompatTest, NativeTransitionColumnTest and StubPlatformStateTest with PlatformLeakGuard
- Release the probe runtime state after DimensionStackSuperflatProbeTest opens a workspace
- Keep core main classes ahead of the core jar on the test classpath so the bytecode gates still scan a directory

core/build.gradle

@@ -29,6 +29,7 @@ import java.nio.charset.StandardCharsets
 plugins {
     id 'java'
     id 'java-library'
+    id 'java-test-fixtures'
     alias(libs.plugins.shadow)
     alias(libs.plugins.slimjar)
 }
@@ -130,6 +131,10 @@ dependencies {
     slim(libs.dom4j)
     slim(libs.jaxen)
 
+    testFixturesImplementation('junit:junit:4.13.2')
+    testFixturesImplementation('org.mockito:mockito-core:5.23.0')
+    testFixturesCompileOnly(libs.paper.api)
+
     testImplementation('junit:junit:4.13.2')
     testImplementation('org.mockito:mockito-core:5.23.0')
     testImplementation(libs.paper.api)
@@ -232,7 +237,7 @@ tasks.named('compileJava', JavaCompile).configure {
 tasks.named('test', Test).configure {
     jvmArgs('--add-modules', 'jdk.incubator.vector')
     dependsOn(languageSourceManifestTask)
-    classpath = files(layout.buildDirectory.dir('generated/language-source')) + files('src/main/resources') + classpath
+    classpath = files(layout.buildDirectory.dir('generated/language-source')) + files('src/main/resources') + sourceSets.main.output + classpath
 }
 
 configurations.matching { it.name.startsWith('slim') }.all { }