chore(ci) restore Codecov support alongside Coveralls

Codecov has a couple features that Coveralls lacks: better PR comments
and PR diff comments of uncovered lines. It's also easier to maintain
compared to the `carryforward` flags of Coveralls.

.github/workflows/job-unit-tests.yml

@@ -146,6 +146,11 @@ jobs:
           flag-name: ${{ steps.lcov.outputs.name }}
           path-to-lcov: './lcov.info'
           parallel: true
+      - name: Codecov Upload
+        if: ${{ !env.ACT && matrix.cc == 'gcc-9' && inputs.coverage }}
+        uses: codecov/codecov-action@v3
+        with:
+          files: './lcov.info'
       - uses: actions/upload-artifact@v2
         if: ${{ failure() && !env.ACT }}
         with:

codecov.yml

@@ -0,0 +1,28 @@
+# https://docs.codecov.io/docs/codecovyml-reference
+codecov:
+  require_ci_to_pass: yes
+  notify:
+    wait_for_ci: yes
+    after_n_builds: 9
+
+coverage:
+  precision: 4
+  round: down
+  range: "75...100"
+  status:
+    project:
+      default:
+        target: auto
+        threshold: 3%
+        base: auto
+        flags:
+          - unit
+        paths:
+          - src
+
+comment:
+  layout: "reach, diff, files"
+  behavior: default
+  require_changes: false
+  require_base: no
+  require_head: yes