t/Makefile: say the default target upfront

t/Makefile: say the default target upfront

Similar to how 2731d048 (Makefile: say the default target upfront.,
2005-12-01) added the default target to the very beginning of the
main Makefile to prevent a random rule that happens to be defined
first in an included makefile fragments from becoming the default
target, protect this Makefile the same way.

This started to matter as we started to include config.mak.uname
and that included makefile fragment does more than defining Make
macros, unfortunately.

Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>