tests: don't run atf_* in a subshell

Shell limitation is that a classic function call via $() is a subshell
and atf-sh(3) commands won't work as epxected there. Subsequently,
atf_skip inside a function won't skip a test. The test will fail later.

A working approach is to pass desired variable name as argument to
a function and don't run subshell.

Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D42646
Fixes: ea82362219ee715cfbb195b2114e73fdc8599fa5