Port codecov improvements matrix-react-sdk -> matrix-js-sdk (#2258)

Author: andybalaam

.github/codecov.yml

@@ -1,3 +1,9 @@
+codecov:
+  allow_coverage_offsets: True
+coverage:
+  status:
+    project: off
+    patch: off
 comment:
   layout: "diff, files"
   behavior: default

.github/workflows/test_coverage.yml

@@ -6,14 +6,21 @@ on:
 jobs:
   test-coverage:
     runs-on: ubuntu-latest
+    env:
+      # This must be set for fetchdep.sh to get the right branch
+      PR_NUMBER: ${{github.event.number}}
     steps:
       - name: Checkout code
         uses: actions/checkout@v2
 
+      - name: Yarn cache
+        uses: c-hive/gha-yarn-cache@v2
+
       - name: Run tests with coverage
         run: "yarn install && yarn build && yarn coverage"
 
       - name: Upload coverage
         uses: codecov/codecov-action@v2
         with:
+          fail_ci_if_error: false
           verbose: true