test: add duplicate class exclusion for gapic-generator-java artifact (#5962)

* Adds an exclusion for the gapic-generator-java artifact in assertUniqueClasses check. 
* BOMContentTest currently fails since inclusion of this artifact in gapic-generator-java-bom as of v2.18.0. gapic-generator-java is not intended for use by client libraries, and was added for downstream usage by the spring-cloud-gcp code generator.

Author: emmileaf

tests/src/test/java/com/google/cloud/BomContentTest.java

@@ -146,6 +146,13 @@ private static void assertUniqueClasses(List<Artifact> allArtifacts)
         continue;
       }
 
+      if (currentArtifact.getGroupId().equals("com.google.api")
+          && currentArtifact.getArtifactId().equals("gapic-generator-java")) {
+        // Skip gapic-generator-java artifact, which is part of gapic-generator-java-bom
+        // but not intended to be client library user-facing. It has copied classes of its dependencies in it.
+        continue;
+      }
+
       String artifactCoordinates = Artifacts.toCoordinates(currentArtifact);
 
       for (String className : classPathEntry.getFileNames()) {