Refactoring of the TextTest class

[stevegrunwell]

This PR applies some minor refactoring to the TextTest class, making it easier to understand what's happening:

1. Add a new buildGenerator() method, which creates a new instance of the text generator at the start of each test and saves it in the private $generator property — this saves a few lines of setup for each test method.
2. Instead of a foreach loop within testTextMaxLength(), use PHPUnit's data providers to explicitly run each scenario as a separate test.
3. For test methods that are using @expectException, explicitly add a failure message to the test to describe why the exception should have occurred.

[pimjansen]

@stevegrunwell im pretty happy with the change. Is it possible to rebase it with latest so we can do some final checks?

[stevegrunwell]

@pimjansen A fresh copy has been pushed. I've also moved the @expectedException annotations to $this->setExpectedException() as the annotation version was deprecated in PHPUnit 8.

[pimjansen]

@stevegrunwell builds are failing, please review

[stevegrunwell]

@pimjansen Apologies, type-hints and ::class resolution in some of the test methods were causing it to fail on old versions of PHP. Everything should be green, now.

[fzaninotto]

Thanks!