Add an integration test for node deletion

[aaronfern]

What this PR does / why we need it:
This PR add an integration test to verify if machine deletion happens when node deletion is triggered

Which issue(s) this PR fixes:
Fixes partially #1123

Special notes for your reviewer:

Release note:

Added an integration test to validate machine deletion when it's corresponding node is deleted

[gardener-prow]

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[thiyyakat]

Thanks for the changes! The test looks good to me. I just had a few questions about possible behaviour.