It looks like we're struggling to find people to test TBB in a timely manner. I certainly can't find time to test during the regular working week any more (life changes ...).
A suggestion: if it's possible to delay they release, then announce the test versions on a Friday for release on a Monday. This should allow at least a day for testing, taking into account timezones, and may help if others are in my position. Or just announce testing when ready but schedule the release for a Monday.
That being said, it's rare for a new release to be buggy so maybe these tests are not as required as initially thought. And it also means that urgent releases can be confidently rolled out with no or limited testing.