[Rename] build.gradle in test module (#403)

This commit refactors the build.gradle file in the test module from o.e.test to
o.opensearch.test namespace.

Signed-off-by: Nicholas Walter Knize <nknize@apache.org>

test/build.gradle

@@ -18,5 +18,5 @@
  */
 
 subprojects {
-  group = 'org.elasticsearch.test'
+  group = 'org.opensearch.test'
 }