test: Simplify usage of realpath in DiffTest #756

Job	Run time
Lint CS	29s
AutoReview Tests	23s
Unit Tests (PHP 8.1, locked)	23s
Wait for the PHAR & Docker images	5m 8s
Unit Tests (PHP 8.1, lowest)	28s
Unit Tests (PHP 8.1, highest)	22s
Update the RequirementChecker	0s
Unit Tests (PHP 8.2, highest)	34s
Unit Tests (PHP 8.3, highest, --ignore-platform-req=php+)	43s
End-to-End Tests (_test_e2e_pass_no_config_min_composer_php, ghcr.io/box-project/box_php725)	1s
End-to-End Tests (_test_e2e_pass_no_config_min_box_php, ghcr.io/box-project/box_php81)	1s
End-to-End Tests (_test_e2e_pass_complete_min_composer_php, ghcr.io/box-project/box_php725)	1s
End-to-End Tests (_test_e2e_pass_complete_min_box_php, ghcr.io/box-project/box_php81)	1s
End-to-End Tests (_test_e2e_fail_complete_min_composer_php, ghcr.io/box-project/box_php725)	1s
End-to-End Tests (_test_e2e_fail_complete_min_box_php, ghcr.io/box-project/box_php81)	1s
End-to-End Tests (_test_e2e_skip_min_composer_php, ghcr.io/box-project/box_php725)	1s
End-to-End Tests (_test_e2e_pass_complete_requirement_checker_not_disabled, ghcr.io/box-project/b...	1s
End-to-End Tests (_test_e2e_fail_conflict, ghcr.io/box-project/box_php81)	1s
RequirementChecker status	2s
	8m 41s

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