feat: use best practices, add tests, fixes #8

[stasadev]

The Issue

• Fixes Tests are failing #8

How This PR Solves The Issue

See https://ddev.com/blog/ddev-add-on-maintenance-guide/#recommendations-for-add-on-maintainers

This PR:

• Runs the add-on update checker curl -fsSL https://ddev.com/s/addon-update-checker.sh | bash
• Adds tests that don't actually check if ddev screenshot works, but perform different checks
• Adds DDEV version constraint >= v1.24.10
• Preserves .ddev/diffy-worker/.env when the add-on is reinstalled
• Switches from $DDEV_HOSTNAME in ddev screenshot to ${DDEV_SITENAME}.${DDEV_TLD} (because DDEV_HOSTNAME can look like test.ddev.site,extra.ddev.site,one-more.ddev.site when you have additional_hostnames)

P.S. the tests can be improved later to see if the ddev screenshot really works.

Manual Testing Instructions

Review https://github.com/stasadev/ddev-diffy/blob/20260226_stasadev_diffy/README.md

ddev add-on get https://github.com/DiffyWebsite/ddev-diffy/tarball/refs/pull/9/head
ddev restart

Automated Testing Overview

Release/Deployment Notes

[stasadev]

Warning

This PR adds .github/workflows/tests.yml, which makes me the primary recipient for email notifications. Please disable and re-enable the workflow after merging it to restore yourself as the recipient.