Fix the build of some ATF tests.

Fix the build of some ATF tests.

When building various programs from a single Makefile, program-specific
variables are of the form <VAR>.<PROG>, not <VAR>_<PROG>.  Fix this
obvious typo to fix the build when WITH_TESTS=yes.

I am not sure how this ever worked before given that manual inspection
of bsd.progs.mk clearly shows that the expected character between the
two components is a dot and not an underscore... but I suspect the
changes in r258095 exposed this oddity.

Approved by:	rpaulo (mentor)