git-notes: Run partial expensive test everywhere

git-notes: Run partial expensive test everywhere

The git-notes expensive timing test is only expensive because it
either did 10,100,1k and 10k iterations or nothing.

Change it to do 10 by default, with an option to run the expensive
version with the old GIT_NOTES_TIMING_TESTS=ZomgYesPlease variable.

Since nobody was ostensibly running this test under TAP the code had
bitrotted so that it emitted invalid TAP. This change fixes that.

The old version would also mysteriously fail on systems without
/usr/bin/time, there's now a check for that using the multiple test
prerequisite facility.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>