Add PHPUnit test for wp_ob_end_flush_all function #866

Job	Run time
Core running from build / Core running from build / Linux	2m 0s
Core running from src / Core running from src / Linux	1m 50s
Core running from build / Core running from build / Windows	3m 33s
Core running from src / Core running from src / Windows	3m 31s
Core running from build / Core running from build / MacOS	4m 10s
Core running from src / Core running from src / MacOS	3m 2s
Gutenberg running from build / Gutenberg running from build / Linux	7m 12s
Gutenberg running from src / Gutenberg running from src / Linux	7m 28s
Gutenberg running from build / Gutenberg running from build / Windows	11m 8s
Gutenberg running from src / Gutenberg running from src / Windows	10m 50s
Gutenberg running from build / Gutenberg running from build / MacOS	14m 33s
Gutenberg running from src / Gutenberg running from src / MacOS	16m 30s
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	1h 25m 49s

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