Issue #3318313: Make OverwriteExistingPackagesValidatorTest fixture less

package_manager/tests/fixtures/overwrite_existing_validation/active/vendor/composer/installed.json

-{
-  "_readme": [
-    "See \\Drupal\\Tests\\package_manager\\Kernel\\OverwriteExistingPackagesValidatorTest::testNewPackagesOverwriteExisting()."
-  ],
-  "packages": []
-}