Test

Introduce integration tests and run them in GH actions #195

Job	Run time
Unit Test: PHP 7.3	21s
Unit Test: PHP 7.4	20s
Unit Test: PHP 8.0	33s
Unit Test: PHP 8.1	30s
Unit Test: PHP 7.2	1m 36s
Unit Test: PHP 8.2	1m 51s
Integration Test: PHP 7.2 \| WP 6.2 (+ ms)	55s
Integration Test: PHP 7.3 \| WP trunk (+ ms)	1m 7s
Integration Test: PHP 7.4 \| WP latest	51s
Integration Test: PHP 8.0 \| WP 6.2	59s
Integration Test: PHP 8.1 \| WP latest (+ ms)	1m 19s
Integration Test: PHP 8.2 \| WP 6.3 (+ ms)	59s
coveralls-finish	7s
	11m 28s

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