smoketest: T6510: make verification of migrated configurations mandatory

For every configuration that is tested from smoketest/configs, there must be
a matching smoketest/config-tests file that has all the set commands
after the migration.

Example of missing testcases:

vyos@vyos:~$ /usr/bin/vyos-configtest
Generating tests
Loaded migration result test for config "basic-api-service"
Loaded migration result test for config "basic-vyos"
Missing migration result test for config "bgp-azure-ipsec-gateway"

... aborted