Run modded and buildSrc tests

- Add a root moddedTest that runs adapters/modded-common through the Fabric build
- Make check depend on moddedTest, ordered ahead of the loader jars
- Gate the buildSrc jar on the buildSrc tests so they stop being skipped
- Drop an unreachable classpath guard from SchemaBuilderPlatformIsolationTest

buildSrc/build.gradle

@@ -18,6 +18,12 @@ repositories {
     }
 }
 
+// Nothing else asks buildSrc for `test`: Gradle only requests the jar that carries the build logic.
+// Gating the jar on the tests is what makes them run at all, and it costs nothing once they are green.
+tasks.named('jar').configure {
+    dependsOn(tasks.named('test'))
+}
+
 dependencies {
     implementation(gradleApi())
     implementation('org.ow2.asm:asm:9.10.1')