Increase our unit test coverage

I am opening this issue as a general issue to attract hacktoberfestians, but we can later open sub-issues to be more specific on what types of testes and where we want them.

Generally we want tests for our main controller (responsible for our reconciliation loop) or/and our backends (responsible for implementing our interface to external secrets solutions support).

Example:

This is the Google Secrets Manager Backend implementation: pkg/gsm/backend.go

This is where its tests are implemented: pkg/gsm/backend_test.go

Our controller logic: controllers/secrets/externalsecret_controller.go

Its tests: controllers/secrets/externalsecret_controller_test.go

I will try to add a checklist here later. But generally if you implement a meaningful test in these places they will be welcomed.

You can also checkout codecov to see where we have less coverage to bump it up a bit.