Update outdated class name in "How to Write a Checker" test example

[pylbrecht]

pylint.testutils.Message got first renamed to pylint.testutils.TestMessage in 49776ac and finally renamed to pylint.testutils.MessageTest in 24cbf8c.

Documentation was not updated to use the new class name.

Type of Changes

	Type
✓	📜 Docs

Description

[pylbrecht]

This is still a WIP until I finish all these open tasks for submitting a PR:

• Add yourself to CONTRIBUTORS if you are a new contributor.
• Add a ChangeLog entry describing what your PR does.
• Write a good description on what the PR does.

[coveralls]

Pull Request Test Coverage Report for Build 1535144340

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.538%

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Totals
Change from base Build 1535104011:	0.0%
Covered Lines:	14026
Relevant Lines:	14995

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💛 - Coveralls

[pylbrecht]

I'm a bit unsure if and how to go about adding an entry to ChangeLog.
Is it necessary in this case? I only see "What's new?" enries and I don't feel this tiny documentation update belongs there.

// edit:

Nevermind, I just looked through your contributing document, which tells me a changelog entry is not necessary in this case:

Add a short entry to the ChangeLog describing the change, except for internal implementation only changes. Not usually required, but for changes other than small bugs we also add a couple of sentences in the release document for that release, (What's New section). For the release document we usually write some more details, and it is also a good place to offer examples on how the new change is supposed to work.

[Pierre-Sassoulas]

It's ok if there is no changelog for this one, thank you for making the documentation better and congrat on becoming a contributor :) For the next MR you'll be trusted enough to run the github action without approval 😉

[DanielNoord]

I did not know about this part of the docs. Seems like we might want to update it as it would be a source of new unittests instead of functional tests. @Pierre-Sassoulas

The change itself looks good though. Thanks! 😄