Update README.md for unit tests

[astridx]

Pull Request for Issue # .

Summary of Changes

Added the description for a file that needs to be changed.

Testing Instructions

Code review

[astridx]

Thank you Richard. I just saw that something else was wrong.

[richard67]

I have tested this item ✅ successfully on 55244cf

1. Code review: Ok.
2. Real test: Without a phpunit.xml, database tests fail because no database found. After proceeding as adviced in this PR, unit tests worked.

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[brianteeman]

I have tested this item ✅ successfully on 55244cf

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[richard67]

RTC

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[wilsonge]

Thanks!

[richard67]

@astridx @wilsonge Sorry, seems I was sleeping when reviewing this PR. I've found some issues, see #32935 . Please review.