No issue: Add threshold to codecov

[sblatz]

Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry or does not need one
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

[colintheshots]

This should be a good change I hope. We can try to find a good percentage to use over time.

[codecov-io]

Codecov Report

Merging #4751 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master   #4751      +/-   ##
===========================================
- Coverage       6.4%    6.4%   -0.01%     
  Complexity      154     154              
===========================================
  Files           205     205              
  Lines          9003    9012       +9     
  Branches       1339    1339              
===========================================
  Hits            577     577              
- Misses         8387    8396       +9     
  Partials         39      39

Impacted Files	Coverage Δ	Complexity Δ
...ug/java/org/mozilla/fenix/DebugFenixApplication.kt	0% <0%> (ø)	0% <0%> (ø)	⬇️

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