atf: Guard against multiple evaluation.

Note that the ATF-C++ macros have the same issue, but they are not as
easily fixed.

MFC after: 3 days
Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D45148

(cherry picked from commit a7beca6fb113986839de73b7cf73d933464898c6)
(cherry picked from commit 745ee870031e3b35141fe720bdebe9baa234e942)

Approved by: re (cperciva)