Enable code coverage.

[kylebrowning]

Attempting to enable code coverage for async-kit

Checklist

• [x] Circle CI is passing (code compiles and passes tests).
• [x] There are no breaking changes to public API.
• [x] New test cases have been added where appropriate.
• [x] All new code has been commented with doc blocks ///.
• [x] The linuxMain.swift is regenerated using swift test --generate-linuxmain

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@094dad1). Click here to learn what that means. The diff coverage is n/a.

@@           Coverage Diff            @@
##             master     #58   +/-   ##
========================================
  Coverage          ?   92.8%           
========================================
  Files             ?      13           
  Lines             ?     584           
  Branches          ?       0           
========================================
  Hits              ?     542           
  Misses            ?      42           
  Partials          ?       0

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Continue to review full report at Codecov.

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[0xTim]

@kylebrowning is there any particular reason you did the codecov step as a separate job instead of just adding it to the existing bionic one?

[kylebrowning]

@0xTim It was originally a part of the bionic, but just incase the step failed, and since they are run in parallel, @tanner0101 suggested to move to another step, and I had no preference.