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.