Handle PHPStan baseline additions about vfs

Whilst I fully agree with @mondrake in #3, I think we can dance around the problem with MR !10097.

However, whilst checking if that MR is hitting \Drupal\Core\Update\UpdateRegistry::getAvailableUpdateFunctions with the same input as before (which it did), I found the test \Drupal\Tests\Core\Update\UpdateRegistryTest::testGetPendingUpdateFunctionsWithLoadedModulesButNotEnabled also passes without these lines:

    // Preload modules to ensure that ::getAvailableUpdateFunctions filters out
    // not enabled modules.
    include_once 'vfs://drupal/sites/default/modules/module_a/module_a.post_update.php';
    include_once 'vfs://drupal/sites/default/modules/module_b/module_b.post_update.php';

So the preloading doesn't seem to make any difference. (See )

Unsure what would be the next step here, but let's put this up for a review and take it from there.

Actually: testGetPendingUpdateFunctionsWithLoadedModulesButNotEnabled()

this is dating back to 2013 - but is "loaded modules but not enabled" still a thing today?