minor #4670 Change PHPUnit link to avoid redirect to homepage (xelaris)

This PR was merged into the 2.3 branch.

Discussion
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Change PHPUnit link to avoid redirect to homepage

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

The previous link is redirected to the homepage of phpunit.de. The new one takes to the intended page.

Commits
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230d275 Change PHPUnit link to avoid redirect to homepage

contributing/code/tests.rst

@@ -112,5 +112,5 @@ browser.
     The code coverage only works if you have Xdebug enabled and all
     dependencies installed.
 
-.. _install PHPUnit: http://www.phpunit.de/manual/current/en/installation.html
+.. _install PHPUnit: https://phpunit.de/manual/current/en/installation.html
 .. _`Composer`: http://getcomposer.org/