fix tests - ClientSettings shouldnt try to get default credentials if it doesnt need em

Change-Id: I9924d7b9f1cf40b81e11a66030109c877eba6a75

Author: igorbernstein2

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/api/ClientSettings.java

@@ -50,6 +50,10 @@ public abstract static class Builder {
     abstract ClientSettings autoBuild(); // not public
 
     public ClientSettings build() {
+      if (getChannelProvider() != null) {
+        return autoBuild();
+      }
+
       Credentials creds;
       try {
         creds = GoogleCredentials.getApplicationDefault();