Fix coverage reporting when tests fail; also push to codecov

[cspotcode]

codecov has path rewriting built-in so we do not need a custom ./scripts/rewrite-coverage-paths.js.

It might not exhibit the weird issues I'm seeing with coveralls' Github Actions integration.

Merging to master so we can get some coverage data from master into Codecov and see if it behaves nicely on pull requests.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@9840cb1). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1047   +/-   ##
=========================================
  Coverage          ?   73.57%           
=========================================
  Files             ?        4           
  Lines             ?      507           
  Branches          ?      130           
=========================================
  Hits              ?      373           
  Misses            ?       83           
  Partials          ?       51           

Flag	Coverage Δ
#node_10	71.59% <0.00%> (?)
#node_12	71.99% <0.00%> (?)
#node_13	73.37% <0.00%> (?)
#node_14	73.37% <0.00%> (?)
#typescript_2_7	72.97% <0.00%> (?)
#typescript_latest	72.18% <0.00%> (?)
#typescript_next	71.99% <0.00%> (?)
#ubuntu	73.37% <0.00%> (?)
#windows	73.37% <0.00%> (?)

---

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[coveralls]

Coverage decreased (-1.2%) to 81.743% when pulling c6b4eef on ab/fix-coverage-when-tests-fail into 9840cb1 on master.

[coveralls]

Coverage remained the same at 82.988% when pulling 4086680 on ab/fix-coverage-when-tests-fail into 9840cb1 on master.