Ignore the coverage plugin in codecov

Local coverage runs won't report on this file, as configured, but
codecov is still flagging it as uncovered code.

Author: sirosen

.codecov.yml

@@ -8,4 +8,7 @@ codecov:
 
 comment: false # avoid spamming reviews
 
+ignore:
+  - "tests/_plugins/_coverage/*"
+
 ...