I would agree it is a healer test, being a healer myself and main-ing WHM ever since the start.

It tests your ability to handle a stressful situation where your ENTIRE party is taking damage at the same time (with the occasional additional accidental hit taken by a DPS, and tank getting the usual hits), so you have to make good use of WHM's skills, and your own situational awareness of the battle + your party member's HP situation.

For your case specifically, you'd find this dungeon a whole lot easier if you just keep regen up on everyone during the 1st and final boss fight where everyone has those DoTs on them. Pay attention to anyone getting more than 3 stacks and pray hard they know how to eat the fruit, or else you might have to pop additional cures on them.

A general rule is NOT to dps, and there's many reasons why you shouldnt at this point. (1) You're still new to the dungeon and it'll be wiser to familiarize with all its mechanics, (2) You're still familiarizing with WHM, and finally (3) You're a healer, not a dps.. so it'd be wiser to focus on what you need to do first, and only perform your secondary role when the primary's comfy (and also it'll help if point 1 and 2 is not an issue for you anymore in the weeks/months ahead).

Oh and also, you can type a short post, then edit it to make it go beyond the 1000 char limit, I just did that for this post