Automatic merge from submit-queue (batch tested with PRs 62025, 63851, 64077, 63967, 63991). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add unit tests to `kubeadm upgrade diff` and small improvements

**What this PR does / why we need it**:

has a couple of commits:

I.
```
1) Store the io.Writer and pass it to sub-commands in upgrade.go
2) Check if the manifest path is an empty string in diff.go:runDiff()
3) Use the io.Writer that upgrade.go defines instead of writing to
os.Stdout directly.
```
II.
```
Add the file diff_test.go, which has a single test:
  TestRunDiff

The test covers most error cases for the runDiff() function,
and also performs a valid diff.
```


**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes kubernetes/kubeadm#826

**Special notes for your reviewer**:
@liztio @luxas 

**Release note**:

```release-note
NONE
```