docs/devel/testing: Split the Avocado documentation into a separate file

The main testing documentation file got very overloaded already.
Thus let's split the Avocado information into a separate file.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20240830133841.142644-43-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
3 files changed