pytest samba-tool visualize: extend colour tests for $NO_COLOR

pytest samba-tool visualize: extend colour tests for $NO_COLOR

As described at https://no-color.org/, the NO_COLOR environment
variable is a widely used defacto-ish standard for asking for no
colour. If someone goes

                NO_COLOR=whatever samba-tool ...

we want to assume they want no ANSI colour codes, as if they had used
--color=no. But first we want to test that, so here we are.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>