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

adapters/neoforge/build.gradle

@@ -132,6 +132,7 @@ configurations.testRuntimeClasspath.extendsFrom(configurations.runtimeClasspath,
 dependencies {
     testImplementation('junit:junit:4.13.2')
     testImplementation('org.mockito:mockito-core:5.23.0')
+    testImplementation(testFixtures('art.arcane:core'))
     testImplementation(libs.spigot) {
         transitive = false
     }