Establish testing approach for KIM and KIM related components: setup e2e tests

Description

Goal is to establish required testing capabilities for KIM and its related components.

In scope is:

Unit-testing for KIM and extenders
Integration tests of KIM using ENV-tests to simulate CR-changes etc.
~Testing logic to compare generated Shoot-Specs of KIM with provide Shoot-Spec templates or Shoot-Specs generated by provisioner etc-~. -> covered by #185
End-2-End tests for the Shoot-Spec migrator component (generates RuntimeCRs using existing Shoot-Specs / Provisioner DB data etc.) to ensure the generated RuntimeCR are including all data.

AC:

[x] Unit-Test framework for KIM business logic established and executed for each PR
- [ ] Integration tests for KIM via ENV-test / Gomega framework are supported
[x] Test logic for generated Shoot-Specs is integrated into KIM (KIM is able to compare its generated Shoot-Specs with provided Shoot-Spec templates or by the Shoot-Specs created by the Provisioner) -> https://github.com/kyma-project/infrastructure-manager/issues/185
[ ] End2End test case available which verifies that KIM produces correct Shoot-Specs using these RuntimeCRs (verify what KIM produces with existing Shoot-Specs)

kyma-project / infrastructure-manager