refactor is_email() tests to use separate data files and PHPUnit dataProvider

[salcode]

There are a number of open tickets involving email validation that seem as though they could benefit from making the tests on is_email() a little more robust.

This is my first pass at that.

Ultimately, I think moving the dataProvider out of IsEmail.php would be helpful so it could be re-used when adding tests for sanitize_email(), but I wanted to take this one step at a time.

I'm relatively new to automated testing and very open to suggestions.

Related: #17433, #17491, #21730, #25108, #30039

[swissspidy]

Using data providers makes sense, though separate files are probably not needed for that.

This PR updates the is_email() tests to improve structure and maintainability by utilizing PHPUnit's dataProvider

Trac ticket: https://core.trac.wordpress.org/ticket/31313

[geekofshire]

Hey everyone,

I noticed that this ticket has been open for quite a while, and I wanted to help move it forward. I've refactored the is_email() tests to use dataProvider while keeping all test data in the same file for better maintainability. Let me know if there’s any feedback or if anything else needs to be updated!

Thanks!