Improve fluent-operator test coverage

[jjsiv]

Testing for fluent-operator is more or less limited to unit testing for plugin types and basic e2e tests for Fluentd. We could definitely improve a lot here to avoid bugs in future releases.

These could definitely use some unit tests as an example, but there is more:
https://github.com/fluent/fluent-operator/blob/master/pkg/operator/daemonset.go
https://github.com/fluent/fluent-operator/blob/master/pkg/operator/fluentd-daemonset.go

I will start working on some, but please share your thoughts and ideas as well.

[github-actions]

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