GH Actions: split workflow #145

Job	Run time
QA / Check XML	31s
QA / Test: PHP 5.4 - PHPCompat stable	36s
QA / Test: PHP 7.4 - PHPCompat dev-develop as 10.99.99	18s
QA / Test: PHP latest - PHPCompat stable	15s
	1m 40s

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