t/httpd-corner: fix test on missing curl-config(1)

t/httpd-corner: fix test on missing curl-config(1)

We were misusing our internal context-dependent `require_cmd',
which requires the caller to call `skip' explicitly when a
return value is desired.  So add a new `skip' call for testers
without curl-config(1).  Furthermore, the `SKIP' label was
incorrectly placed for Test::More::skip to use, so create an
explicit block for it.

Reported-by: Jonathan Corbet <corbet@lwn.net>
Link: https://public-inbox.org/meta/878qikobes.fsf@trenco.lwn.net/