Add PHPUnit test for wp_ob_end_flush_all function #1015

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Core running from build / Core running from build / Linux	2m 4s
Core running from src / Core running from src / Linux	1m 57s
Core running from build / Core running from build / Windows	4m 8s
Core running from src / Core running from src / Windows	3m 36s
Core running from build / Core running from build / MacOS	5m 6s
Core running from src / Core running from src / MacOS	3m 37s
Gutenberg running from build / Gutenberg running from build / Linux	8m 2s
Gutenberg running from src / Gutenberg running from src / Linux	8m 11s
Gutenberg running from build / Gutenberg running from build / Windows	11m 15s
Gutenberg running from src / Gutenberg running from src / Windows	11m 42s
Gutenberg running from build / Gutenberg running from build / MacOS	19m 59s
Gutenberg running from src / Gutenberg running from src / MacOS	13m 54s
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	1h 33m 33s

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