Fix Codecov "file not found at ./coverage.txt"

[fxamacker]

Closes #900

Description

Change "cover.txt" to "coverage.txt" in Makefile.

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• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@ede5edf). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #901   +/-   ##
=========================================
  Coverage          ?   39.85%           
=========================================
  Files             ?      254           
  Lines             ?    31521           
  Branches          ?        0           
=========================================
  Hits              ?    12562           
  Misses            ?    18120           
  Partials          ?      839           

Flag	Coverage Δ
unittests	39.85% <0.00%> (?)

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[turbolent]

Thank you for fixing this! 🙏