Site Health: Add a test for PHP default timezone.

The test reports a failure if the default timezone was changed with `date_default_timezone_set()` to anything other than `UTC`.

WordPress historically uses `UTC` as the default timezone for calculating date and time offsets, overriding it is not recommended and can cause widespread and obscure issues.

Props Rarst, Clorith, TimothyBlynJacobs.
Fixes #48692.
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git-svn-id: https://core.svn.wordpress.org/trunk@46597 1a063a9b-81f0-0310-95a4-ce76da25c4cd