adding test cases for increasing coverage

[yoqwerty]

What Changed

Added unit tests in modern.test.ts and common.test.ts for improving the code coverage

Why

https://github.com/intuit/postcss-themed/issues/18 Todo:

• [x] Add tests
• [x] Add docs
• [x] Add yourself to contributors (run yarn all-contributors add [name] [type]) see https://github.com/all-contributors/all-contributors/blob/master/docs/cli/usage.md

Published PR with canary version: 2.8.2-canary.68.784

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@b6dd622). Click here to learn what that means. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master      #68   +/-   ##
=========================================
  Coverage          ?   93.39%           
=========================================
  Files             ?        5           
  Lines             ?      348           
  Branches          ?       92           
=========================================
  Hits              ?      325           
  Misses            ?       11           
  Partials          ?       12           

Impacted Files	Coverage Δ
src/modern/index.ts	91.11% <100.00%> (ø)

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[yoqwerty]

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