Issue #3279229 by phenaproxima, tedbow: StagedProjectsValidatorTest should use a virtual project

...staged_validation/new_project_added/active.installed.json

..._staged_validation/new_project_added/active/composer.json

...staged_validation/new_project_added/staged.installed.json

..._staged_validation/new_project_added/staged/composer.json

...project_staged_validation/no_errors/active.installed.json

.../project_staged_validation/no_errors/active/composer.json

...ixtures/project_staged_validation/no_errors/composer.json

...project_staged_validation/no_errors/staged.installed.json

.../project_staged_validation/no_errors/staged/composer.json

...t_staged_validation/project_removed/active.installed.json

...ct_staged_validation/project_removed/active/composer.json

...t_staged_validation/project_removed/staged.installed.json

...ct_staged_validation/project_removed/staged/composer.json

...t_staged_validation/version_changed/active.installed.json

...ct_staged_validation/version_changed/active/composer.json

...t_staged_validation/version_changed/staged.installed.json

...ct_staged_validation/version_changed/staged/composer.json

...ernel/ReadinessValidation/StagedProjectsValidatorTest.php