t/README: Update "Skipping tests" to align with best practices

t/README: Update "Skipping tests" to align with best practices

The example I initially added to "Skipping tests" wasn't very
good. We'd rather skip tests using the three-arg prereq form to the
test_* functions, not bail out with a skip message.

Change the documentation to reflect that, but retain the bailout
example under a disclaimer which explains that it's probably not a
good idea to use it.

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