Add smoke test for skaffold diagnose

[corneliusweig]

The skaffold diagnose command has bad test coverage (both integration and unit tests). There are reasons why it may not make sense to have full unit test coverage there. However, basic test coverage should be there to check whether the subcommand runs at all.

This PR adds basic smoke tests which check if skaffold diagnose runs successfully. It covers all artifact types that have an example.

[codecov-io]

Codecov Report

Merging #2157 into master will increase coverage by 0.2%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #2157     +/-   ##
=========================================
+ Coverage   56.74%   56.95%   +0.2%     
=========================================
  Files         183      183             
  Lines        7860     7961    +101     
=========================================
+ Hits         4460     4534     +74     
- Misses       2988     3012     +24     
- Partials      412      415      +3

Impacted Files	Coverage Δ
pkg/skaffold/build/cluster/sources/sources.go	87.87% <0%> (-1.46%)	⬇️
pkg/skaffold/build/cluster/kaniko.go	36.27% <0%> (-0.87%)	⬇️
pkg/skaffold/schema/v1beta10/upgrade.go	73.33% <0%> (+6.66%)	⬆️
pkg/skaffold/build/tag/git_commit.go	86.9% <0%> (+6.9%)	⬆️

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[balopat]

Nice!!! Thank you for this!