Add parameter and return type hints to all test methods

@pratik_specbee, please read the issue summary...

The last submitted patch, 6: 3339459-6-type-hints-in-tests.patch, failed testing. View results →

Adding type hints revealed a typing problem in the tests. Needs work to fix this.

We can't type $result or $simplified_result as they can be a string or an array. We can't use union types because we are still supporting

This only changes test files. The changes are in-line with the changes required for the tests to pass with D10, and the tests are passing.

Here's a new patch that starts with #9 and also corrects (and documents) the @return type in MimeMailFormatHelper to include an array in the same way that was done in the test. I think this is still in scope because the whole point of adding type hinting in the tests is to ensure that we're always passing around what we think we're passing around, and since we're not we have to make sure the documentation comments have the right types everywhere.

Committed #11.

Automatically closed - issue fixed for 2 weeks with no activity.