Improve schema upgrade tests

[dgageot]

• Remove some duplication
• Add more assertions on the versions actually tested

[codecov-io]

Codecov Report

Merging #1383 into master will increase coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1383      +/-   ##
==========================================
+ Coverage    44.4%   44.67%   +0.26%     
==========================================
  Files         109      109              
  Lines        4542     4542              
==========================================
+ Hits         2017     2029      +12     
+ Misses       2319     2307      -12     
  Partials      206      206

Impacted Files	Coverage Δ
pkg/skaffold/schema/v1alpha5/config.go	100% <0%> (+50%)	⬆️
pkg/skaffold/schema/v1alpha1/config.go	100% <0%> (+50%)	⬆️
pkg/skaffold/schema/v1beta1/config.go	100% <0%> (+50%)	⬆️
pkg/skaffold/schema/v1alpha4/config.go	100% <0%> (+50%)	⬆️
pkg/skaffold/schema/v1alpha3/config.go	100% <0%> (+50%)	⬆️
pkg/skaffold/schema/v1alpha2/config.go	100% <0%> (+50%)	⬆️

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