tests: avoid tons of spurious failures on NetBSD

tests: avoid tons of spurious failures on NetBSD

* t/ax/am-test-lib.sh (process_requirements): Set the '$am_tool' variable
to the empty string before trying to unset it; otherwise, we might be
attempting to unset an already-unset variable, which (together with the
presence of the 'errexit' shell flag) causes spurious failures at least
with the /bin/sh shell from NetBSD 5.1.  This was actually causing the
great majority of the Automake tests (all those not using a "required=..."
declaration) to fail spuriously on that platform!

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>