runtests.pl: allow -vc point to a separate curl binary to verify with

runtests.pl: allow -vc point to a separate curl binary to verify with

The specified curl binary will then be used to verify the running
server(s) instead of the development version. This is very useful in
some cases when the development version fails to verify correctly as
then the test case may not run at all.

The actual test will still be run with the "normal" curl executable
(unless the test case specifies something differently).