Added codecov.io coverage uploader to CI

refs linear.app/tryghost/issue/CORE-74/improve-the-test-situation

- this commit adds the codecov GitHub Action into CI so we can upload
  coverage reports
- the coverage files need to be in XML for them to work with
  codecov, so this commit also adds cobertura (XML) as a reporter

.github/workflows/test.yml

@@ -118,6 +118,7 @@ jobs:
 
       - name: Unit test coverage
         run: yarn cov:unit
+      - uses: codecov/codecov-action@v2
 
       - uses: daniellockyer/action-slack-build@master
         if: failure() && github.event_name == 'push' && github.ref == 'refs/heads/main'

package.json

@@ -29,7 +29,7 @@
     "test": "mocha --require=./test/utils/overrides.js --exit --trace-warnings --recursive --extension=test.js --timeout=60000",
     "test:all": "yarn test:unit && yarn test:integration && yarn test:e2e && yarn lint",
     "test:debug": "DEBUG=ghost:test* yarn test",
-    "test:unit": "c8 --all -n 'core/{*.js,frontend,server,shared}' --reporter text-summary mocha --require=./test/utils/overrides.js --exit --trace-warnings --recursive --extension=test.js './test/unit' --timeout=2000",
+    "test:unit": "c8 --all -n 'core/{*.js,frontend,server,shared}' --reporter text-summary --reporter cobertura mocha --require=./test/utils/overrides.js --exit --trace-warnings --recursive --extension=test.js './test/unit' --timeout=2000",
     "test:integration": "mocha --require=./test/utils/overrides.js --exit --trace-warnings --recursive --extension=test.js  './test/integration' --timeout=5000",
     "test:e2e": "mocha --require=./test/utils/overrides.js --exit --trace-warnings --recursive --extension=test.js './test/e2e-api' './test/e2e-frontend' './test/e2e-server' --timeout=10000",
     "test:regression": "mocha --require=./test/utils/overrides.js --exit --trace-warnings --recursive --extension=test.js  './test/regression' --timeout=60000",