[Test refactor 3/5] Notification service improvement
This improves the notification service in Slack to provide a more comprehensive view over the tests.

Previously:

<img width="493" alt="image" src="https://user-images.githubusercontent.com/30755778/154765757-8ea3f016-b2df-471b-b161-2a31e67effe2.png">

Now:

<img width="614" alt="image" src="https://user-images.githubusercontent.com/30755778/154765783-dc62a4ad-d85b-42b6-af1b-b9e3914a2b4e.png">

To do before merge:
- Update the slack channel reference so that it posts to the right channel
- Ensure compatibility with the `push` job that runs on each commit to `master`
_The documentation is not available anymore as the PR was closed or merged._<|||||>Lysandre, you're still summing up one+many on top of the new preview (categories) - this is confusing/misleading since most of the time it's the same error that happens on both setups and shouldn't be counted twice.

and why not do the exact same format for the categories? it'd be much easier to read if it's the same format for all groups and sub-groups, no?

and I'd still format it as:

```
1 | 2 | name1
5 | 6 | name2
```
which is much less noisier, but then I don't know how you format it - you said it was complicated.

if the header isn't too much work to add then:
```
1 | 2 | title
--|---|------- 
1 | 2 | name1
5 | 6 | name2
```

<|||||>Formatting it as you offer isn't complicated, it was using pandas that I wasn't too keen on. I personally prefer the format above but it's probably because I have seen it too much by now :) Let's hear what others think.

And indeed, will update the categories at the top.<|||||>The format isn't too important to me - I'll get used to either way quickly I think :-) 

If I would have to choose, I'd prefer @stas00 solution though as well - would be nice to include the "single-gpu | multi-gpu" header every time <|||||>I have updated the notification service to output the following format:

<img width="635" alt="image" src="https://user-images.githubusercontent.com/30755778/155244600-eb95b042-9f3a-4282-b505-16468f93fa68.png">
