[Tests] fix tests due to recent changes in PHP's behavior

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11176
| License       | MIT
| Doc PR        | 

The drawback of this solution is that the pretty printed string produced by ``json_encode()`` isn't validated anymore. Instead, the tests only ensures that the represented object structure is as expected. I guess that's fair enough since the goal of the test is not to test PHP's pretty printing feature.