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/bukkit/plugin/build.gradle

@@ -22,6 +22,7 @@ dependencies {
     testImplementation(libs.paper.api)
     testImplementation(libs.bstats)
     testImplementation(project(':core'))
+    testImplementation(testFixtures(project(':core')))
     testImplementation(volmLibCoordinate) {
         transitive = false
     }