Make modifications to keyless.example deterministic

Mark Andrews requested to merge 3345-fix-keyless-example-generation-v9_16 into v9_16

The perl modifation code for keyless.example was not deterministic (/NXT/ matched part of signature) resulting in different error strings being returned. Replaced /NXT/ with /A RRSIG NSEC/ and updated expected error string,

(cherry picked from commit 69d5e22e)

Closes #3345 (closed)

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