Email: Add unit tests covering email validation and sanitization.

In preparation for later work to allow non-US-ASCII email addresses, this change extends the unit test suite for `is_email()` and adds new tests covering `antispambot()` and `sanitize_email()`.

This work was done collaboratively during WordCamp Vienna, 2026 as a Contributor Challenge in cooperation with and support from ICANN and also GeoTLDs Universal Acceptance Local Initiative.

Developed in: https://github.com/WordPress/wordpress-develop/pull/11552
Discussed in: https://core.trac.wordpress.org/ticket/31992

Props agulbra, akirk, benniledl, dmsnell.
See #31992.


git-svn-id: https://develop.svn.wordpress.org/trunk@62225 602fd350-edb4-49c9-b593-d223f7449a82