Introduce integration tests and run them in GH actions #186

Job	Run time
Integration Test: PHP 7.2 \| WP 6.2 (+ ms)	47s
Integration Test: PHP 7.3 \| WP trunk (+ ms)	53s
Integration Test: PHP 7.4 \| WP latest	48s
Integration Test: PHP 8.0 \| WP 6.2	55s
Integration Test: PHP 8.1 \| WP latest (+ ms)	47s
Integration Test: PHP 8.2 \| WP 6.3 (+ ms)	1m 7s
Unit Test: PHP 7.3	25s
Unit Test: PHP 7.4	31s
Unit Test: PHP 8.0	21s
Unit Test: PHP 8.1	32s
Unit Test: PHP 7.2	1m 43s
Unit Test: PHP 8.2	1m 28s
coveralls-finish	7s
	10m 24s

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