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

ghost/portal/.github/workflows/test.yml

@@ -22,7 +22,9 @@ jobs:
           node-version: ${{ matrix.node }}
       - run: yarn install
       - run: yarn build
-      - run: yarn test
+      - run: yarn test:ci
+
+      - uses: codecov/codecov-action@v2
 
       - uses: daniellockyer/action-slack-build@master
         if: failure() && github.event_name == 'push' && github.ref == 'refs/heads/main'

ghost/portal/package.json

@@ -69,6 +69,7 @@
   },
   "jest": {
     "coverageReporters": [
+      "cobertura",
       "text-summary",
       "html"
     ]