bsdgrep: add some additional tests for fgrep

Previously added tests only check that fgrep is somewhat sane and works. Add
some more tests that check that the implementation is basically functional
and not producing incorrect results with various flags.

Reviewed by: cem, emaste, ngie
Approved by: emaste (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D12056