Add PHPUnit tests for wp_parse_slug_list function #1023

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Core running from build / Core running from build / Linux	1m 56s
Core running from src / Core running from src / Linux	1m 57s
Core running from build / Core running from build / Windows	3m 32s
Core running from src / Core running from src / Windows	3m 19s
Core running from build / Core running from build / MacOS	4m 23s
Core running from src / Core running from src / MacOS	4m 41s
Gutenberg running from build / Gutenberg running from build / Windows	11m 40s
Gutenberg running from build / Gutenberg running from build / Linux	8m 30s
Gutenberg running from src / Gutenberg running from src / Linux	8m 20s
Gutenberg running from src / Gutenberg running from src / Windows	11m 31s
Gutenberg running from build / Gutenberg running from build / MacOS	11m 54s
Gutenberg running from src / Gutenberg running from src / MacOS	10m 53s
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	1h 22m 38s

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