Add PHPUnit tests for wp_maybe_load_widgets function #1014

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Core running from build / Core running from build / Linux	2m 1s
Core running from src / Core running from src / Linux	1m 54s
Core running from build / Core running from build / Windows	3m 54s
Core running from src / Core running from src / Windows	3m 58s
Core running from build / Core running from build / MacOS	4m 12s
Core running from src / Core running from src / MacOS	4m 36s
Gutenberg running from build / Gutenberg running from build / Linux	8m 16s
Gutenberg running from src / Gutenberg running from src / Linux	8m 28s
Gutenberg running from build / Gutenberg running from build / Windows	12m 4s
Gutenberg running from src / Gutenberg running from src / Windows	12m 10s
Gutenberg running from build / Gutenberg running from build / MacOS	15m 23s
Gutenberg running from src / Gutenberg running from src / MacOS	18m 11s
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