Bump phpunit/phpunit from 9.6.13 to 9.6.15 #1302

Job	Run time
pre_job	5s
PHP 7.4 on ubuntu-latest with lowest dependencies	40s
static_code_analysis	43s
PHP 7.4 on ubuntu-latest with locked dependencies	45s
PHP 8.0 on ubuntu-latest with lowest dependencies	49s
PHP 8.0 on ubuntu-latest with locked dependencies	46s
PHP 8.1 on ubuntu-latest with lowest dependencies	48s
PHP 8.1 on ubuntu-latest with locked dependencies	40s
PHP 7.4 on macos-latest with lowest dependencies	1m 29s
PHP 7.4 on macos-latest with locked dependencies	1m 29s
PHP 8.0 on macos-latest with lowest dependencies	1m 39s
PHP 8.0 on macos-latest with locked dependencies	2m 1s
PHP 8.1 on macos-latest with lowest dependencies	1m 26s
PHP 8.1 on macos-latest with locked dependencies	1m 27s
PHP 7.4 on windows-latest with lowest dependencies	47s
PHP 7.4 on windows-latest with locked dependencies	39s
PHP 8.0 on windows-latest with lowest dependencies	48s
PHP 8.0 on windows-latest with locked dependencies	43s
PHP 8.1 on windows-latest with lowest dependencies	43s
PHP 8.1 on windows-latest with locked dependencies	44s
coverage	1m 36s
build_phar	29s
build_docker_image	0s
notify	0s
	21m 16s

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