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

[tobiscr]

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 Implement testing logic to compare the Shoot-spec generated by provisioner with the shoot-spec generated by KIM #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:

• Unit-Test framework for KIM business logic established and executed for each PR
  • Integration tests for KIM via ENV-test / Gomega framework are supported
• 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) -> Implement testing logic to compare the Shoot-spec generated by provisioner with the shoot-spec generated by KIM #185
• End2End test case available which verifies that KIM produces correct Shoot-Specs using these RuntimeCRs (verify what KIM produces with existing Shoot-Specs)

[tobiscr]

See also POC how K8s-test framework could be used: https://github.tools.sap/I539990/poc-k8stestsuite