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kubeadm: add test coverage to completion.go
**What this PR does / why we need it**:
Add `completion_test.go` with the following tests:
- TestNewCmdCompletion
- TestRunCompletion
A separate commit exports the function GetSupportedShells() to obtain the list of supported shells.
Test coverage is at 96%. The only untested bit is an `io.Writer.Write()` call in `RunCompletion()`. in the case of `bytes.Buffer` it would panic and/or always return `nil` for `error`.
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
NONE
**Special notes for your reviewer**:
NONE
**Release note**:
```release-note
NONE
```
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