Disable WP_CRON when installing PHPUnit tests

[Chouby]

While running PHPunit tests for one of my plugins with PHP 8.1, I noticed deprecation notices in the Requests library during the WordPress installation process. These notices reveal in fact that an http request is always fired by a cron task during the WordPress installation.

In bootstrap.php, we can find these lines:

/*
 * Cron tries to make an HTTP request to the site, which always fails,
 * because tests are run in CLI mode only.
 */
define( 'DISABLE_WP_CRON', true );

However the constant is not passed to the install.php script.
I propose to add this constant in this script too.

[SergeyBiryukov]

In 52359:

Build/Test Tools: Disable WP Cron when installing PHPUnit tests.

During the WordPress installation process when running the test suite, an HTTP request was always fired by a cron task and failed, because tests are run in CLI mode only.

To avoid that, the DISABLE_WP_CRON constant was previously added to the bootstrap.php file. However, the constant is not passed to the install.php script. This commit makes a similar change to install.php.

Follow-up to [760/tests], [872/tests].

Props Chouby.
Fixes #54612.