minor symfony#52031 [Notifier] [Telegram] Rework tests (OskarStark)

This PR was squashed before being merged into the 6.4 branch.

Discussion
----------

[Notifier] [Telegram] Rework tests

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | --
| License       | MIT

its better to not write json content by hand

Commits
-------

7e3194b [Notifier] [Telegram] Rework tests

Author: fabpot