test-lib: Make the test_external_* functions TAP-aware

test-lib: Make the test_external_* functions TAP-aware

Before TAP we just ran the Perl test and assumed that it failed if
nothing was printed on STDERR. Continue doing that, but introduce a
`test_external_has_tap' variable which tests can set to indicate that
they're outputting TAP.

If it's set we won't output a test plan, but trust the external test
to do so. That way we can make external tests work with a TAP harness,
but still maintain compatibility with test-lib's own way of tracking
tests through the test-results directory.

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