t/README: reformat Do, Don't, Keep in mind lists

t/README: reformat Do, Don't, Keep in mind lists

The list of Don'ts for test writing has grown large such that it is hard
to see at a glance which section an item is in. In other words, if I
ignore a little bit of surrounding context, the "don'ts" look like
"do's."

To make the list more readable, prefix "Don't" in front of every first
sentence in the items.

Also, the "Keep in mind" list is out of place and awkward, because it
was a very short "list" beneath two very long ones, and it seemed easy
to miss under the list of "don'ts," and it only had one item. So move
this item to the list of "do's" and phrase as "Remember..."

Signed-off-by: Matthew DeVore <matvore@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>