Extend unit tests for Site Health component.

[peterwilsoncc]

A file for site health check unit tests has been added in tests/phpunit/tests/site-health.php.

At the time of writing it only includes tests for various states of cron, expanding these would help with future development of the component.

[Clorith]

Agreed, some inspiration can be taken from the plugin (​https://github.com/WordPress/health-check/tree/master/tests/phpunit), although the tests there also need to be expanded upon as they're fairly limited at this time, and relate more to user experience (in the relation of the Site Health Checks).

I'm going to flag this for 5.3, to get some traction, but with the knowledge it may end up getting bumped to 5.4 as well, and change it to a task as unit tests can be added throughout betas so we don't stop any momentum gained.

[rukkykf]

I'd like to get to work on this. Will take a while though, I'm still working on getting familiar with the codebase.

[desrosj]

This still needs a working patch to be considered. I'm going to move this to future release, but because this just aims to add tests, it can be moved back whenever it's deemed ready.