test(plugin): add tests to prevent some regressions

[ryanclark]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

Quickly add some tests to prevent regressions like #418 from happening again (this will fail CI until that PR is merged)

[codecov]

Codecov Report

Merging #419 into master will increase coverage by 1.82%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #419      +/-   ##
==========================================
+ Coverage   40.63%   42.46%   +1.82%     
==========================================
  Files           3        3              
  Lines         219      219              
  Branches       48       48              
==========================================
+ Hits           89       93       +4     
+ Misses        107      103       -4     
  Partials       23       23

Impacted Files	Coverage Δ
src/karma-webpack.js	46.96% <0%> (+2.02%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 48878eb...dfe0d78. Read the comment docs.