Issue #3344124 by tedbow: Validate fake_site with composer

package_manager/tests/fixtures/fake_site/composer.json

package_manager/tests/fixtures/fake_site/composer.lock

...r/tests/fixtures/fake_site/vendor/composer/installed.json

...er/tests/fixtures/fake_site/vendor/composer/installed.php

...s/fixtures/fake_site/vendor/drupal/core-dev/composer.json

...es/fake_site/vendor/drupal/core-recommended/composer.json

...tests/fixtures/fake_site/vendor/drupal/core/composer.json

...tures/path_repos/cweagans--composer-patches/composer.json

.../tests/fixtures/path_repos/drupal--core-dev/composer.json

...ixtures/path_repos/drupal--core-recommended/composer.json

...ager/tests/fixtures/path_repos/drupal--core/composer.json

package_manager/tests/src/Kernel/FakeSiteFixtureTest.php