t5561: skip tests if curl is not available

t5561: skip tests if curl is not available

It's possible to have libcurl installed but not the curl
command-line utility. The latter is not generally needed for
Git's http support, but we use it in t5561 for basic tests
of http-backend's functionality. Let's detect when it's
missing and skip this test.

Note that we can't mark the individual tests with the CURL
prerequisite. They're in a shared t556x_common that uses the
GET and POST functions as a level of indirection, and it's
only our implementations of those functions in t5561 that
requires curl. It's not a problem, though, as literally
every test in the script would depend on the prerequisite
anyway.

Reported-by: Jens Krüger <Jens.Krueger@frm2.tum.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>