test(vmclass): add discovery handler unit tests

[loktev-d]

Description

• Add unit tests for discoveryCommonFeatures function covering various node configurations
• Add unit tests for Handle function with different nodeSelector configurations

Why do we need it, and what problem does it solve?

What is the expected result?

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: vmclass
type: test 
summary: add discovery handler unit tests