Stop Codecov failing good pull requests

Something doesn't quite work with our Codecov setup. It somehow thinks
that the coverage is decreasing even though it's either not changing or
even increasing.

Until those problems are resolved, let's not mark pull requests as
failed so that we can rely on the pull's overall status report.

.github/codecov.yml

@@ -0,0 +1,5 @@
+coverage:
+  status:
+    project:
+      default:
+        informational: true