Introduce integration tests and run them in GH actions #177

Job	Run time
Unit Test: PHP 7.3	28s
Unit Test: PHP 7.4	31s
Unit Test: PHP 8.0	21s
Unit Test: PHP 8.1	24s
Unit Test: PHP 7.2	1m 33s
Unit Test: PHP 8.2	1m 26s
Integration Test: PHP 7.2 \| WP 6.2 (+ ms)	47s
Integration Test: PHP 7.3 \| WP trunk (+ ms)	1m 1s
Integration Test: PHP 7.4 \| WP latest	52s
Integration Test: PHP 8.0 \| WP 6.2	47s
Integration Test: PHP 8.1 \| WP latest (+ ms)	1m 19s
Integration Test: PHP 8.2 \| WP 6.3 (+ ms)	1m 13s
coveralls-finish	3s
	10m 45s

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