kubeadm: test-cmd: join{config, discovery}

[dmmcquay]

What this PR does / why we need it: Adding test-cmds for kubeadm join {config, discovery}. Will followup with more test-cmds for other flags as soon as validation for the flags works.

Adding tests is a WIP from #34136

Special notes for your reviewer: /cc @luxas @pires

Release note:

NONE

[k8s-reviewable]

This change is 

[luxas]

/lgtm
/approve

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

The following people have approved this PR: dmmcquay, luxas

Needs approval from an approver in each of these OWNERS Files:

• cmd/kubeadm/OWNERS [luxas]

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[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 41216, 41362, 41275, 41277, 41412)