Date/Time: Make `get_permalink()` more resilient against PHP timezone changes.

Overriding default PHP timezone with `date_default_timezone_set()`, while not recommended, should not inadvertently result in changing existing permalinks.

Add a unit test.

Props Rarst, steevithak, archon810, maciejmackowiak, Ov3rfly, Cybr, hometowntrailers, scvleon, miette49.
Fixes #48623.
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git-svn-id: https://core.svn.wordpress.org/trunk@46595 1a063a9b-81f0-0310-95a4-ce76da25c4cd