t/README: Document the do's and don'ts of tests

t/README: Document the do's and don'ts of tests

Add a "Do's, don'ts & things to keep in mind" subsection to the
"Writing Tests" documentation. Much of this is based on Junio C
Hamano's "Test your stuff" section in
<7vhbkj2kcr.fsf@alter.siamese.dyndns.org>.

I turned it into a list of do's and don'ts to make it easier to skim
it, and integrated my note that a TAP harness will get confused if you
print "ok" or "not ok" at the beginning of a line.

Thad had to be fixed in 335f87871fe5aa6b3fd55b2b4e80f16fe9681483 when
TAP support was introduced.

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