test defs: avoid spurious XPASS results with NetBSD make

test defs: avoid spurious XPASS results with NetBSD make

* tests/defs (make_can_chain_suffix_rules): Ensure to properly
declare the '.SUFFIXES:' in the temporary Makefile used to check
whether $MAKE is able to automatically chain suffix rules.  This
avoids spurious XPASS results with (at least) NetBSD make in some
'suffix*.tap' tests; in a sense also, this also makes the probing
of $MAKE capabilities more faithful, since the Automake-generated
Makefiles are expected to properly declare the '.SUFFIXES:' as
well.