test-lib: Print missing prerequisites in test output

test-lib: Print missing prerequisites in test output

Change the test output to print needed prerequisites as part of the
TAP. This makes it easy to see at a glance why a test was
skipped. Before:

    ok 7 # skip <message>
    ok 9 # skip <message>

After:

    ok 7 # skip <message> (prereqs: DONTHAVEIT)
    ok 9 # skip <message> (prereqs: HAVEIT,DONTHAVEIT)

This'll also be useful for smoke testing output, where the developer
reading the output may not be familiar with the system where tests are
being skipped.

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