Don't leave `path` behind when executing `:chflags_success`

Don't leave `path` behind when executing `:chflags_success`

Prior to this change a `SF_IMMUTABLE` chflagsat(2)'ed file (`path`) was left
behind, which sabotaged kyua(1) from being able to clean up the work directory,
This resulted in unnecessary work for folks having to clean up the work
directory on non-disposable systems, which defaults to `/tmp`. Use `UF_OFFLINE`
instead of `SF_IMMUTABLE`, in part because setting `SF_IMMUTABLE` isn't relevant
to the test and `SF_IMMUTABLE` cannot be cleared at all securelevels, as pointed
out by @asomers.

Additional work is required to catch cases like this upfront in the future to
avoid tester headache. See PR # 247765 for more details/followup.

Suggested by:	asomers
Reviewed By:	asomers, #tests
MFC after:	1 week
PR:		247761
Sponsored by:	DellEMC
Differential Revision: https://reviews.freebsd.org/D25561