Revert "disable unit tests until they work"

This reverts commit 606cc456d2.

plugins/cloud-gce/pom.xml

@@ -32,9 +32,6 @@ governing permissions and limitations under the License. -->
         <tests.rest.suite>cloud_gce</tests.rest.suite>
         <tests.rest.load_packaged>false</tests.rest.load_packaged>
         <xlint.options>-Xlint:-rawtypes,-unchecked</xlint.options>
-        <!-- TODO: re-enable unit tests once they work
-             https://github.com/elastic/elasticsearch/issues/13623 -->
-        <skip.unit.tests>true</skip.unit.tests>
     </properties>
 
     <dependencies>