Fix TestDNS_ServiceLookup_ARecordLimits so that it only creates test …

…agents the minimal amount of time (#21608)

* get rid of unused column

* get rid of duplicate section now that deletion of unused column makes the section duplicate..

* explicit set protocol rathern than infer it in checkDNSService

* explicit have attribute for whether to set EDNS0 in the test cases  rathern than infer it in checkDNSService

* now modify so that test agents are only created for each unique configuration which is based on the a_record_limit.

* Fix TestDNS_ServiceLookup_AnswerLimits  so that it only creates test agents the minimal amount of time. (#21609)

Fix TestDNS_ServiceLookup_AnswerLimits  so that it only creates test agents the minimal amount of time