While we might never be able to have 100% of our code strict, we can at
the very least do it for all of our tests. This ensures that our tests
are using our API with the types as intended by the test author, and not
silently be cast to what our API requires.
* Resolve Phpstan Messages Reader/Xls Shared/Escher
Reduce number of Phpstan messages by addressing their issues.
* Scrutinizer
Hyperactivity.
* More Scrutinizer
Getting close.
* Even More Scrutinizer
Closer and closer.