tests/multiboot: Fix whitespace failure

Commit b43671f8 accidentally broke run_test.sh within tests/multiboot;
due to a subtle change in whitespace.

These two commands produce theh same output (at least, for sane $IFS
of space-tab-newline):

echo -e "...$@..."
echo -e "...$*..."

But that's only because echo inserts spaces between multiple arguments
(the $@ case), while the $* form gives a single argument to echo with
the spaces already present.

But when converting to printf %b, there are no automatic spaces between
multiple arguments, so we HAVE to use $*.

It doesn't help that run_test.sh isn't part of 'make check'.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
