distros.conf: switch 'testing' to 'stretch', deactivate it for stable

deactivate it to avoid having a possibly wrong hardcoded definition of
'testing' in 'stable'

the *only* effect is that piuparts will use a separate basetgz for 'testing'
and not reuse a 'codename'd one

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: sleep a bit if basetgz creation is in progress or failed

Signed-off-by: Andreas Beckmann <anbe@debian.org>
drop .sh suffix from example script

Signed-off-by: Andreas Beckmann <anbe@debian.org>