Add PHPUnit tests for wp_widgets_add_menu function #1016

Job	Run time
Core running from build / Core running from build / Linux	1m 56s
Core running from src / Core running from src / Linux	2m 6s
Core running from build / Core running from build / Windows	3m 29s
Core running from src / Core running from src / Windows	3m 6s
Core running from build / Core running from build / MacOS	4m 52s
Core running from src / Core running from src / MacOS	4m 29s
Gutenberg running from build / Gutenberg running from build / Linux	7m 59s
Gutenberg running from src / Gutenberg running from src / Linux	8m 20s
Gutenberg running from build / Gutenberg running from build / Windows	11m 38s
Gutenberg running from src / Gutenberg running from src / Windows	11m 46s
Gutenberg running from build / Gutenberg running from build / MacOS	14m 10s
Gutenberg running from src / Gutenberg running from src / MacOS	15m 20s
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	1h 29m 14s

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