Suricata

If we look at https://app.codecov.io/gh/OISF/suricata/blob/master/src/detect-filemagic.c
we will notice that some code parts, such as the prefilter are not covered by our tests.

This ticket's work includes:
- Identifying other filemagic code portions that also need coverage
- Adding suricata-verify tests with rules that make use of those
- Checking if we need to update our userguide documentation on those keywords, and improve it accordingly

Please note that in order to test filemagic it is necessary to build Suricata with support for this feature.