Instead of failing the tests, we should rather skip them if ncat is
not available.
While we're at it, also mention ncat in the README.md file as a
requirement for the migration tests.

Resolves: https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/issues/4


Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211221092130.444225-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>