[DPE-4911]: Fix flaky relations test

[Gu1nness]

Issue

The new_relations test fails sometimes because it fails to get the secret. This happens when one of the units has not received the RelationJoined event. It ends up in this state

Solution

Ensuring that the message is good allows to ensure that the secret is set without modifying the test application. This is done like it's done already in another test.

Miscellaneous

• Bump library version in test application
• Add missing check when creating the operator user
• Improve test base on json serialization