chore: add test-jar goal in core sub-project

PiperOrigin-RevId: 886245655

Author: tilgalas

core/pom.xml

@@ -229,6 +229,16 @@
       <plugin>
         <artifactId>maven-compiler-plugin</artifactId>
       </plugin>
+      <plugin>
+        <artifactId>maven-jar-plugin</artifactId>
+        <executions>
+          <execution>
+            <goals>
+              <goal>test-jar</goal>
+            </goals>
+          </execution>
+        </executions>
+      </plugin>
       <plugin>
         <artifactId>maven-surefire-plugin</artifactId>
         <executions>