Test

Introduce integration tests and run them in GH actions #190

Job	Run time
Unit Test: PHP 7.3	25s
Unit Test: PHP 7.4	27s
Unit Test: PHP 8.0	21s
Integration Test: PHP 7.2 \| WP 6.2 (+ ms)	46s
Unit Test: PHP 8.1	20s
Integration Test: PHP 7.3 \| WP trunk (+ ms)	54s
Unit Test: PHP 7.2	1m 39s
Integration Test: PHP 7.4 \| WP latest	55s
Unit Test: PHP 8.2	1m 29s
Integration Test: PHP 8.0 \| WP 6.2	45s
Integration Test: PHP 8.1 \| WP latest (+ ms)	1m 13s
Integration Test: PHP 8.2 \| WP 6.3 (+ ms)	1m 30s
coveralls-finish	23s
	11m 7s

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