Add integration tests

[timebertt]

What would you like to be added:

Integration tests based on envtest for controllers, webhooks, library, etc.

• Add integration test for controllerring controller #487
• Add integration test for sharder controller #506
• Add integration test for shardlease controller #488
• Add integration test for sharder webhook #489
• Add integration test for shard controller #490
• Add integration test for shard lease #497

Why is this needed:

While working on the thesis, I didn't add any integration tests for the sake of finishing the implementation part of the thesis.
Now that the project is supposed to mature and get more stable, having good test coverage is crucial.