Add codecov settings

.codecov.yml

@@ -0,0 +1,33 @@
+# Override defaults for codecov.io checks.
+#
+# Codecov's defaults seem to leave red marks in GitHub CI checks in a
+# rather arbitrary manner, probably because computers are bad with
+# floating point numbers.
+#
+# Documentation is at https://docs.codecov.io/docs/codecov-yaml;
+# reference is at https://docs.codecov.io/docs/codecovyml-reference.
+#
+# To validate this file, use:
+#
+#   curl --data-binary @.codecov.yml https://codecov.io/validate
+
+# Allow coverage percentage a precision of zero decimals, and round to
+# the nearest number (for example, 89.957 to to 90; 89.497 to 89%).
+# Coverage above 90% is good, below 80% is bad.
+coverage:
+  round: nearest
+  range: 80..90
+  precision: 0
+
+  # Aim for a target test coverage of 90% in codecov/project check (do
+  # not allow project coverage to drop below that), and allow
+  # codecov/patch a threshold of 1% (allow coverage in changes to drop
+  # by that much, and no less).  That should be good enough for us.
+  status:
+    project:
+      default:
+        target: 90%
+        threshold: 1%
+    patch:
+      default:
+        threshold: 1%