Test inline-signing with $INCLUDE

Dan found an issue that the isc.org zone did not work anymore. Specifically the zone has inline-signing marked and uses $INCLUDE statements.

$INCLUDE statements are relative to the working directory, not the zone file.

1. Add a test for this, or add a comment to this issue where case this is tested.
2. Document how $INCLUDE works (for example the ARM could at least mention from where relative paths are searched).
3. Logging for missing $INCLUDEs also needs some love