tests/Makefile had a mix of TAB vs. 8-space indentation; given
that it is a Makefile, TAB is more idiomatic even though in these
particular cases the choice of whitespace didn't matter.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>