[YAML] resolve variables in inlined YAML

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11665
| License       | MIT
| Doc PR        | 

#11569 does not resolve variables in inline YAML.
The change was triggered by [this fixture file](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Yaml/Tests/Fixtures/sfMergeKey.yml). I also extracted it into its own test.