EntityResource: Provide comprehensive test coverage for ShortcutSet entity

The patch contains two additional changes:
1.

+++ b/core/core.link_relation_types.yml
...
+customize-form:
+  description: "The target IRI points to a resource where a submission form for customizing associated resource can be obtained."
+  reference: '[RFC6861]'

Without this change we have problem here:

# EntityResourceTestBase.php:440
Failed asserting that Array &0 (
    0 => 'https://drupal.org/link-relations/delete-form'
    1 => 'edit-form'
    2 => 'collection'
) is identical to Array &0 (
    0 => 'customize-form'
    1 => 'https://drupal.org/link-relations/delete-form'
    2 => 'edit-form'
    3 => 'collection'
).

2.

+++ b/core/modules/shortcut/src/ShortcutSetAccessControlHandler.php
@@ -19,6 +19,8 @@ class ShortcutSetAccessControlHandler extends EntityAccessControlHandler {
    */
   protected function checkAccess(EntityInterface $entity, $operation, AccountInterface $account) {
     switch ($operation) {
+      case 'view':
+        return AccessResult::allowedIf($account->hasPermission('access shortcuts'))->cachePerPermissions();

Without this change we have problem here:

# EntityResourceTestBase.php:388
Failed asserting that 403 is identical to 200.

Thank you @Wim Leers for review and @Yogesh Pawar for help! A small additional trivial cleanup. (I was so happy that the tests passed, that I sent the patch faster than necessary, sorry for this).

Added @todo in getNormalizedPostEntity, by analogy with the other config cases. Please, point me this is right change or we need some other change? (sorry for the eternal flaws)

Random fail.

#19: I'm not sure about s/IRI/URI/. Looks like IRI - Internationalised Resource Identifier. But maybe URI makes sense too.

customize-form:
  description: "The target IRI points to a resource where a submission form for customizing associated resource can be obtained."
  reference: '[RFC6861]'

This is copy from

edit-form:
  description: "The target IRI points to a resource where a submission form for editing associated resource can be obtained."
  reference: '[RFC6861]'

because it have similar behavior, which looks like in the scope of the RFC6861 specification.

#21: Tried to do it.