Update in-repo PHPUnit documentation

[q0rban]

The current PHPUnit documentation that lives in /tests/phpunit/README.txt is out of date. It contains incorrect testing steps, and should ideally be in Markdown like other in-repo documentation. It could also benefit from a link to the more detailed ​handbook page.

Correct the in-repo PHPUnit testing instructions, and switch to Markdown.

Trac ticket: https://core.trac.wordpress.org/ticket/62065

@ironprogrammer thank you for the review! Apologies for the delay in my addressing your feedback. I believe I've got your suggestions addressed, however I did run into a slight issue switching to backticks while preserving proper indentation. Let me know if you would like me to try to format this in a different way.

I couldn't resist fixing the typo. Good catch!

[ironprogrammer]

This update LGTM 👍🏻 Moving into 6.8 milestone for consideration.

[desrosj]

Copying my review message here to make the status obvious on Trac:

I left some suggestions, but while reviewing I realized we're creating an additional place for our documentation to become out of date.

What if we included the intro paragraph and then just linked out to the handbook for setup instructions? Is there a reason we must include setup instructions in this file?

Removing trunk version as this is not a new bug.

[peterwilsoncc]

[...] I realized we're creating an additional place for our documentation to become out of date.

What if we included the intro paragraph and then just linked out to the handbook for setup instructions? Is there a reason we must include setup instructions in this file?

I like this suggestion, in [55670] the security policy in the repository was updated to refer developers to Hacker1 as the source of truth. I think it would be good to treat the handbook as the source of truth for running tests.

[desrosj]

This is not critical to the 6.8 release, so I'm going to move it so that the feedback can be addressed.