Add PHPUnit tests for wp_parse_slug_list function #1030

Job	Run time
Core running from src / Core running from src / Linux	1m 56s
Core running from build / Core running from build / Linux	2m 1s
Core running from build / Core running from build / Windows	4m 4s
Core running from src / Core running from src / Windows	3m 29s
Core running from build / Core running from build / MacOS	3m 13s
Core running from src / Core running from src / MacOS	4m 32s
Gutenberg running from build / Gutenberg running from build / Linux	7m 53s
Gutenberg running from src / Gutenberg running from src / Linux	8m 14s
Gutenberg running from build / Gutenberg running from build / Windows	11m 35s
Gutenberg running from src / Gutenberg running from src / Windows	12m 26s
Gutenberg running from build / Gutenberg running from build / MacOS	18m 57s
Gutenberg running from src / Gutenberg running from src / MacOS	11m 25s
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	1h 29m 48s

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