Add code coverage badge + output on tests.

[jafin]

• Add codecoverage badge + output on tests.
• bump test dependencies

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@6d61573). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #254   +/-   ##
=========================================
  Coverage          ?   25.32%           
=========================================
  Files             ?      257           
  Lines             ?    26207           
  Branches          ?     3181           
=========================================
  Hits              ?     6638           
  Misses            ?    19042           
  Partials          ?      527           

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[ststeiger]

Merged.