tests/dockerfiles: unify debian-toolchain references

We use the debian release number elsewhere so fix it for consistency
along with the broken comment.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230228190653.1602033-15-alex.bennee@linaro.org>
1 file changed