Bug-compatible refactor of docker-env + output tests

[tstromberg]

The previous tests for docker-env did not provide much value, causing issues like #6524 to be both easy to introduce, and difficult to confidently fix.

This PR refactors docker-env to be trivially tested, by allowing it take only strings and a writer as inputs. This means we no longer have to mock out parts of libmachine, and we can now compare outputs.

This PR introduces no behavior changes or new tests. A follow-up PR will fix the following bugs I discovered:

• If --unset and --no-proxy are used together, the output is corrupted
• cmd, powershell, and emacs add a trailing tab to the script
• cmd and powershell output contain an invalid newline
• unset output has incorrect comment

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tstromberg

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[medyagh]

Ah. Sorry about this mistake. I need to make windows test failures look like others ASAP

[medyagh]

/ok-to-test

[minikube-pr-bot]

Error: running mkcmp: exit status 1

[minikube-pr-bot]

All Times minikube: [ 93.982657 95.527866 92.615017]
All Times Minikube (PR 6527): [ 94.924589 97.897852 94.996733]

Average minikube: 94.041847
Average Minikube (PR 6527): 95.939725

Averages Time Per Log

+----------------------+-----------+--------------------+
|         LOG          | MINIKUBE  | MINIKUBE (PR 6527) |
+----------------------+-----------+--------------------+
| minikube v           |  0.127620 |           0.117489 |
| Creating kvm2        | 19.882912 |          19.857527 |
| Preparing Kubernetes | 49.387085 |          50.674750 |
| Pulling images       |  4.002997 |           3.457292 |
| Launching Kubernetes | 19.100134 |          19.904683 |
| Waiting for cluster  |  0.053628 |           0.071570 |
+----------------------+-----------+--------------------+

[tstromberg]

PTAL, I've repurposed this PR for cleanup-only work.

[minikube-pr-bot]

All Times Minikube (PR 6527): [ 95.758353 96.993859 96.684549]
All Times minikube: [ 96.778772 94.904052 96.517588]

Average minikube: 96.066804
Average Minikube (PR 6527): 96.478920

Averages Time Per Log

+----------------------+-----------+--------------------+
|         LOG          | MINIKUBE  | MINIKUBE (PR 6527) |
+----------------------+-----------+--------------------+
| minikube v           |  0.135747 |           0.123767 |
| Creating kvm2        | 19.697606 |          20.190388 |
| Preparing Kubernetes | 50.369065 |          49.980214 |
| Pulling images       |  3.862927 |           3.752001 |
| Launching Kubernetes | 20.259151 |          20.815988 |
| Waiting for cluster  |  0.055089 |           0.061804 |
+----------------------+-----------+--------------------+

[codecov-io]

Codecov Report

Merging #6527 into master will decrease coverage by 0.04%.
The diff coverage is 58.03%.

@@            Coverage Diff             @@
##           master    #6527      +/-   ##
==========================================
- Coverage   37.95%   37.91%   -0.05%     
==========================================
  Files         138      137       -1     
  Lines        8708     8683      -25     
==========================================
- Hits         3305     3292      -13     
+ Misses       4986     4978       -8     
+ Partials      417      413       -4

Impacted Files	Coverage Δ
cmd/minikube/cmd/env.go	57.81% <58.03%> (+0.89%)	⬆️
cmd/minikube/cmd/start.go	22.1% <0%> (-0.2%)	⬇️
pkg/minikube/bootstrapper/bootstrapper.go	0% <0%> (ø)	⬆️

[medyagh]

Read half of it, looks very nice ! reading the rest

[minikube-pr-bot]

All Times minikube: [ 94.496072 94.742602 90.992936]
All Times Minikube (PR 6527): [ 94.248907 94.779100 94.299592]

Average minikube: 93.410537
Average Minikube (PR 6527): 94.442533

Averages Time Per Log

+----------------------+-----------+--------------------+
|         LOG          | MINIKUBE  | MINIKUBE (PR 6527) |
+----------------------+-----------+--------------------+
| minikube v           |  0.136897 |           0.119398 |
| Creating kvm2        | 19.937192 |          19.884683 |
| Preparing Kubernetes | 50.257349 |          49.650694 |
| Pulling images       |           |           4.019329 |
| Launching Kubernetes | 21.281739 |          19.176318 |
| Waiting for cluster  |  0.237086 |           0.074410 |
+----------------------+-----------+--------------------+