docs: small improvements in testing/overview.rst

[kenjis]

Description

• small improvements

I wanted to add info for testing beginners like the following in the Build Your First Application:

If you are not familiar with PHP, we recommend that you check out the W3Schools PHP Tutorial before continuing.

But I can't find good proper URLs.

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide