tests: add LIBCURL prerequisite to tests needing libcurl

tests: add LIBCURL prerequisite to tests needing libcurl

Add and use a LIBCURL prerequisite for tests added in
6dcbdc0d661 (remote: create fetch.credentialsInUrl config,
2022-06-06).

These tests would get as far as emitting a couple of the warnings we
were testing for, but would then die as we had no "git-remote-https"
program compiled.

It would be more consistent with other prerequisites (e.g. PERL for
NO_PERL) to name this "CURL", but since e9184b0789a (t5561: skip tests
if curl is not available, 2018-04-03) we've had that prerequisite
defined for checking of we have the curl(1) program.

The existing "CURL" prerequisite is only used in one place, and we
should probably name it "CURL_PROGRAM", then rename "LIBCURL" to
"CURL" as a follow-up, but for now (pre-v2.37.0) let's aim for the
most minimal fix possible.

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