fix: test: test-syncplugin.c:plugin_register was broken

The variables example2com, example3com, and example4com where not
being initalised on all paths leading to Coverity issues ID 637690,
ID 637691 and ID 637692 being raised.  In addition the tests to free
them were wrong as they depended on the unintialised variables. This
has been fixed.

Closes #5598

Merge branch '5598-properly-determine-if-name-is-dynamic' into 'main'

See merge request isc-projects/bind9!11155