netbsd-tests: Update a test case to chase grep symlink handling changes

This test case verifies that grep detects symlink loops when traversing
a directory hierarchy.

Fixes: fc12c191c087 ("grep: Default to -p instead of -S.")
Reviewed by: ngie, jhb
Reported by: Jenkins
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D46544

(cherry picked from commit a700bef1e4ee3e6f4e1a86a374bf9b4044f69a70)