fix kubeadm unit tests (testdata)

[prezha]

ref: #9694
ref: #9693

currently, bsutil (kubeadm) unit tests have hardcoded list of versions to test, referencing testdata subdirectories for relevant k8s version to test, and it has to be manually updated with each new version released, eg:

minikube/pkg/minikube/bootstrapper/bsutil/kubeadm_test.go

Line 80 in 9b96485

versions := []string{"v1.20.0-beta.1", "v1.19", "v1.18", "v1.17", "v1.16", "v1.15", "v1.14", "v1.13", "v1.12"}

and

minikube/pkg/minikube/bootstrapper/bsutil/kubeadm_test.go

Line 111 in 9b96485

versions := []string{"v1.20.0-beta.1", "v1.19", "v1.18", "v1.17", "v1.16", "v1.15", "v1.14", "v1.13", "v1.12"}

we also need to cover the situation where we still don't have the <major>.<minor>.0 stable version released (eg, as with 'v1.20.0-beta.1'), in which case we should use that latest k8s version until stable version gets released in test data files

therefore, it would be good to:

• have a dynamic (auto-discover) list of k8s versions to test (hack update: generate unit tests for new k8rs version #9708 will automatically create new testdata set for newer versions), while also
• have only one set (subdirectory) with test data for each <major>.<minor> k8s version under pkg/minikube/bootstrapper/bsutil/testdata/

[prezha]

/assign

[medyagh]

this is a good idea