Correct Gradle plugin dependency groupId in spark-test-bom/pom.xml to match Gradle convention.

Author: CoderYellow

spark-test-bom/pom.xml

@@ -34,8 +34,11 @@
                 <version>${bigdata-test.version}</version>
             </dependency>
 
+            <!-- Gradle plugin marker artifact: by Gradle convention its groupId is the plugin id
+                 itself (org.openprojectx.bigdata-test, with a hyphen), NOT the .core groupId. It is
+                 published at org/openprojectx/bigdata-test/org.openprojectx.bigdata-test.gradle.plugin/. -->
             <dependency>
-                <groupId>org.openprojectx.bigdata.test.core</groupId>
+                <groupId>org.openprojectx.bigdata-test</groupId>
                 <artifactId>org.openprojectx.bigdata-test.gradle.plugin</artifactId>
                 <version>${bigdata-test.version}</version>
             </dependency>