fix: test: test-syncplugin treats firstlbl as a prefix, not an exact label

The label length and the string length where not being checked so
a label that started with a string that matched the skip label would
incorrectly match.

Closes #6212

Merge branch '6212-fix-syncplugin-test-driver' into 'main'

See merge request isc-projects/bind9!12365