qapi: Consistent whitespace in tests/Makefile

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: NEric Blake <eblake@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>