Issue #3327753 by kunal.sachdev, tedbow, Wim Leers: Create a helper function to add listeners

...ager/tests/src/Kernel/ComposerExecutableValidatorTest.php

...ager/tests/src/Kernel/ComposerJsonExistsValidatorTest.php

...manager/tests/src/Kernel/ComposerPatchesValidatorTest.php

...anager/tests/src/Kernel/ComposerSettingsValidatorTest.php

package_manager/tests/src/Kernel/DiskSpaceValidatorTest.php

...ager/tests/src/Kernel/EnvironmentSupportValidatorTest.php

package_manager/tests/src/Kernel/LockFileValidatorTest.php

package_manager/tests/src/Kernel/MultisiteValidatorTest.php

...manager/tests/src/Kernel/PackageManagerKernelTestBase.php

package_manager/tests/src/Kernel/SettingsValidatorTest.php

package_manager/tests/src/Kernel/StageEventsTest.php

package_manager/tests/src/Kernel/StageOwnershipTest.php

package_manager/tests/src/Kernel/StageTest.php

package_manager/tests/src/Kernel/StatusCheckTraitTest.php

package_manager/tests/src/Kernel/SymlinkValidatorTest.php

...ager/tests/src/Kernel/WritableFileSystemValidatorTest.php

tests/src/Kernel/CronUpdaterTest.php

tests/src/Kernel/ReadinessCheckTest.php

tests/src/Kernel/StatusCheck/CronServerValidatorTest.php

.../Kernel/StatusCheck/StagedDatabaseUpdateValidatorTest.php