Add tests for path_join

[karlijnbok]

Adds unit tests for path_join in build/wp-includes/functions.php

Trac ticket: https://core.trac.wordpress.org/ticket/55897#ticket

Two things I'd like to mention about the path_join method:

1. Maybe some verification or checks should be added to make sure $base and $path are both indeed strings. Newer php versions might break otherwise (PHP8 for instance can give a fatal error).
2. The ltrim for the path variable is redundant, because if the path variable starts with slashes, it will be considered absolute, thus entering the if statement and returning $path. It will only reach the ltrim( $path, '/' ) if $path is not absolute, i.e. does not start with a slash, so nothing needs to be trimmed.

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[github-actions]

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[SergeyBiryukov]

Thanks for the PR! Merged in https://core.trac.wordpress.org/changeset/53457.

[jrfnl]

Two things I'd like to mention about the path_join method:

1. Maybe some verification or checks should be added to make sure $base and $path are both indeed strings. Newer php versions might break otherwise (PHP8 for instance can give a fatal error).
2. The ltrim for the path variable is redundant, because if the path variable starts with slashes, it will be considered absolute, thus entering the if statement and returning $path. It will only reach the ltrim( $path, '/' ) if $path is not absolute, i.e. does not start with a slash, so nothing needs to be trimmed.

Well done @karlijnbok 👍🏻 and I concur on both your observations.