zfs tests: Silence clang warning

"assigning to 'pattern_t *' from 'const pattern_t *' discards qualifiers"

Reviewed by: asomers
Reported by: clang
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42791

(cherry picked from commit 5b36076d28ad1920b178da93d667dcfeae426494)