curl: skip FTP tests in run-ptest

curl: skip FTP tests in run-ptest

It looks like something related to FTP in curl, be it the protocol itself
or the harness, is unstable under load.  We've been seeing random failures
in automated QA, and Debian does too.

Until this issue is resolved, disable all of the FTP tests on the hope
that this is the underlying common factor.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>