When switching themes, `wp_map_nav_menu_locations()` is used to ensure nav menus are placed in the relevant menu location. Occasionally, menus are registered to locations with numeric slugs, rather than strings. `wp_map_nav_menu_locations()` assumed it would be the latter, and ran `stripos()` on those numeric slugs. This behavior is deprecated in PHP 7.3.
As this is the last PHP 7.3 error in unit tests, this commit also removes PHP 7.3 from Travis' `allowed_failures` list.
Props pento, desrosj, jorbin.
Merges [43899] to trunk.
See #45018.
git-svn-id: https://develop.svn.wordpress.org/trunk@44167 602fd350-edb4-49c9-b593-d223f7449a82
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