Make awk regular expressions more precise in the "dnssec" system test

The /IN *SOA/ regular expression can give a false positive match. For
example, the following line matches, which is unexpected for the test:

    example.				      60 IN DNSKEY	257 3 13 oCFm3P5uMtWuqXqbyUUlHHNgjCztzASAFhXUlnnudcYkYmNwOwYm6jah RewD1COujT/Lj2+JyKDV3sFRrINSOA==

It detects the "INSOA" sub-string in the base64-encoded data.

Change the regular expression to make it more precise.