Bump phpunit/phpunit from 9.6.13 to 9.6.15 #90

Job	Run time
buster / build	18m 40s
bookworm / build	18m 42s
bullseye / build	19m 18s
buster / test (pi, run_installation_tests.sh)	3m 4s
buster / test (pi, run_installation_tests2.sh)	3m 1s
buster / test (pi, run_installation_tests3.sh)	3m 56s
buster / test (hans, run_installation_tests.sh)	54s
buster / test (hans, run_installation_tests2.sh)	1m 32s
buster / test (hans, run_installation_tests3.sh)	58s
bookworm / test (pi, run_installation_tests.sh)	3m 0s
bookworm / test (pi, run_installation_tests2.sh)	2m 50s
bookworm / test (pi, run_installation_tests3.sh)	3m 26s
bookworm / test (hans, run_installation_tests.sh)	1m 2s
bookworm / test (hans, run_installation_tests2.sh)	54s
bookworm / test (hans, run_installation_tests3.sh)	53s
bullseye / test (pi, run_installation_tests.sh)	3m 2s
bullseye / test (pi, run_installation_tests2.sh)	3m 3s
bullseye / test (pi, run_installation_tests3.sh)	3m 56s
bullseye / test (hans, run_installation_tests.sh)	55s
bullseye / test (hans, run_installation_tests2.sh)	54s
bullseye / test (hans, run_installation_tests3.sh)	53s
buster / cleanup	0s
bookworm / cleanup	0s
bullseye / cleanup	0s
	1h 34m 53s

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