tests/fusefs: appease gcc -Wcast-qual

Fix gcc build error.

Reviewed by: asomers, kib
Fixes: 7e68af7ce2c1 ("fusefs: redo vnode attribute locking")
Differential Revision: https://reviews.freebsd.org/D56370
(cherry picked from commit 0c09c6527b16953d355125bee7a839f743a39dad)