fix bsutil (kubeadm) unit tests (testdata)

[prezha]

fixes: #9707

this pr replaces the hard-coded list of k8s versions to test and builds it dynamically from the existing testdata versions subdirectories, and also handles the case when <major>.<minor>.0 stable version is not yet released (eg, as with 'v1.20.0-beta.1') by using the latest k8s pre-release version available until stable version gets out, so to have only one set (subdirectory) with test data for each <major>.<minor> k8s version under pkg/minikube/bootstrapper/bsutil/testdata/

[k8s-ci-robot]

Hi @prezha. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[minikube-bot]

Can one of the admins verify this patch?

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: medyagh, prezha

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