Test

Introduce integration tests and run them in GH actions #191

Job	Run time
Integration Test: PHP 7.2 \| WP 6.2 (+ ms)	47s
Unit Test: PHP 7.3	31s
Integration Test: PHP 7.3 \| WP trunk (+ ms)	1m 7s
Unit Test: PHP 7.4	20s
Integration Test: PHP 7.4 \| WP latest	1m 13s
Unit Test: PHP 8.0	29s
Integration Test: PHP 8.0 \| WP 6.2	50s
Unit Test: PHP 8.1	28s
Integration Test: PHP 8.1 \| WP latest (+ ms)	44s
Unit Test: PHP 7.2	1m 37s
Integration Test: PHP 8.2 \| WP 6.3 (+ ms)	1m 0s
Unit Test: PHP 8.2	1m 33s
coveralls-finish	5s
	10m 44s

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