oeqa/runtime/scp: Disable scp test for dropbear

oeqa/runtime/scp: Disable scp test for dropbear

Fedora is switching to use sftp as the backend for scp. This means the
scp test fails on Fedora 36 hosts with a dropbear target as dropbear
doesn't support sftp. This change is in the upstream openssh code, other
distros have not yet changed the default but probably will follow.

The easiest way to resolve test failures in dropbear images is to stop
testing this against dropbear as it is no longer expected to work and will
likely spread as the change filters through other distros.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>