[CI] move test coverage job as a separate job and disable Codecov status comment

[jimmylai]

Summary

move test coverage job as a separate job and change Codecov status as patch mode which only report percentage comes from current PR changes (rather than focus on project status).

Test Plan

jimmylai-mbp:LibCST jimmylai$ curl --data-binary @codecov.yml https://codecov.io/validate
Valid!

{
  "coverage": {
    "status": {
      "project": false,
      "patch": true
    }
  }
}

[codecov-io]

Codecov Report

Merging #48 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #48   +/-   ##
=======================================
  Coverage   95.78%   95.78%           
=======================================
  Files         112      112           
  Lines        6745     6745           
=======================================
  Hits         6461     6461           
  Misses        284      284

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