make tests ignorable with "make -i"

make tests ignorable with "make -i"

Allow failed tests to be ignored using make's "-i". The patch also
disables parallel make in t/. This doesn't make the testing any
different as before: the tests were run sequentially before.

It also allows to run more tests, ignoring the ones usually failing
just to figure out if something else broke.  (Or to ignore plainly
uninteresting situations because of the testing being done on say...
cygwin ;)

Signed-off-by: Junio C Hamano <junkio@cox.net>