During a release we track the status of testing images as we go. This is coordinated in IRC on irc.debian.org #debian-cd.
Link to the test pages should be added here:
Buster (Debian 10)
Stretch (Debian 9)
2019-09-08 9.11
2019-09-07 9.10
2019-04-27 9.9
2019-02-16 9.8
2018-11-10 9.6
2018-07-14 9.5
2018-03-10 9.4
2017-12-09 9.3
2017-10-07 9.2
2017-07-22 9.1
2017-06-17 9.0
Jessie (Debian 8)
2018-06-23 8.11
2017-12-09 8.10
2017-07-23 8.9
2016-06-04 8.5
2016-04-03 8.4
2015-09-05 8.2
2015-06-06 8.1
2015-04-25 8.0
Wheezy (Debian 7)
Squeeze (Debian 6)
2011-02-09 6.0