Backport of Fix TestDNS_ServiceLookup_ARecordLimits so that it only c…

…reates test agents the minimal amount of time into release/1.19.x (#21612)

* backport of commit 0ca8552

* backport of commit ff2713b

* backport of commit f8c8d0a

* backport of commit da2c449

* backport of commit 38e824a

* backport of commit 8cf76bd

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Co-authored-by: John Murret <john.murret@hashicorp.com>