Add PHPUnit tests for wp_parse_id_list function #1029

Job	Run time
Core running from src / Core running from src / Linux	2m 41s
Core running from build / Core running from build / Linux	4m 0s
Core running from build / Core running from build / Windows	4m 19s
Core running from src / Core running from src / Windows	3m 41s
Core running from build / Core running from build / MacOS	5m 36s
Core running from src / Core running from src / MacOS	3m 34s
Gutenberg running from build / Gutenberg running from build / Linux	8m 11s
Gutenberg running from src / Gutenberg running from src / Linux	8m 32s
Gutenberg running from build / Gutenberg running from build / Windows	11m 36s
Gutenberg running from src / Gutenberg running from src / Windows	11m 27s
Gutenberg running from build / Gutenberg running from build / MacOS	11m 50s
Gutenberg running from src / Gutenberg running from src / MacOS	21m 42s
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	1h 37m 13s

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