pkg/testutil: add in-memory Plugin builder

[ahmetb]

Introducing builder-pattern methods to create in-memory Plugin/Platform objects
that has good defaults, but can be overwritten for test purposes. Readability
of the test files (e.g. validation/*_test.go) now speak for themselves.

The testutil.NewPlugin() returns a valid index.Plugin instance. Any tests
expected to fail can modify its values with the builder pattern. (ditto for
testutil.NewPlatform()).

• had to move validation code to pkg/index/validation due to a cyclic
  import situation that occurred. This should be fine.
• renamed some test cases for ease of readability
• I've thought about adding code validation for **_test.go to not to have any
  index.Plugin{ or index.Platform{ strings. But this might be an overkill.

Fixes #270. Beware of merge conflicts that will raise due to in-flight PRs.

[codecov-io]

Codecov Report

Merging #283 into master will decrease coverage by 0.15%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
- Coverage   55.07%   54.91%   -0.16%     
==========================================
  Files          19       19              
  Lines         877      874       -3     
==========================================
- Hits          483      480       -3     
  Misses        341      341              
  Partials       53       53

Impacted Files	Coverage Δ
pkg/receiptsmigration/migration.go	48.14% <100%> (ø)	⬆️
pkg/index/validation/validate.go	95% <100%> (ø)
cmd/validate-krew-manifest/main.go	58.33% <100%> (ø)	⬆️
pkg/index/indexscanner/scanner.go	69.56% <50%> (ø)	⬆️

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[corneliusweig]

This reads very nicely. Well done!

There are some left-over Plugin usages in util_test.go and platform_test.go which could be cleaned up. Did you leave those out intentionally?

[ahmetb]

There are some left-over Plugin usages in util_test.go and platform_test.go which could be cleaned up. Did you leave those out intentionally?

Yeah it seems like I forgot.
Perhaps we can even explore adding a check for preventing index.Plugin{} or index.Platform{} initializations and have all tests use this approach.

[ahmetb]

platform_test.go

So looks like platform.go can't use this utility package, because it uses a cyclic import when used in test case Test_matchPlatformToSystemEnvs().

I'm planning to move this method to pkg/installation as pkg/index.PluginSpec isn't the best place to have this method. I'll do this post this PR (#284 to track).

[corneliusweig]

/lgtm
/approve

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