343 industries have placed a halt on public testing through the Halo Insider's program for The Master Chief Collection, stating that testing won't be resumed until 'it's ready'.
The FireFight horde test was supposed to land for Halo Insiders back in July, but never actually happened, and the company has been quiet since, with just a short explanation stating that they were working on solved bugs in the system and some complications with the Halo testing system hierarchy.
The bugs were considered 'blocking bugs' - meaning they were so severe it would prevent meaningful gameplay and testing. Although you might expect a beta to have bugs, the company explained that a blocking bug is a more serious type of issue;
Keep in mind that though we address, mitigate and solve 'blocking bugs' for each ring of flighting, this doesn’t mean that any flight is bug-free. In fact, it’s quite the contrary since each flight is very much a work-in-progress development build.
Microsoft tend to use rings in their testing methods, and each build has to be tested at each ring without a blocking bug to progress to the next, with the public rings normally being the highest rings in that stack.
The first two rings are internal, but the third ring is the public Halo Insider group. As the build intended for testing has failed to get past the internal rings, it cannot be made public.
When we announced MCC coming to PC and Reach coming to MCC, we thought we knew exactly what the roadmap was to bring about the best game for gamers. We were wrong about a few things and this is something that happens often in the complex, fluid world of game development. To compensate for this, schedules around flighting had to be shifted out, but, even with these challenges, the development team has overcome them and is continuing to work on it day-in and day-out towards bringing MCC to PC and Halo: Reach to MCC the right way.
So while there's no date yet for the expected testing, it will be coming, eventually.