Push method code coverage bar to 50 percent

This change adds or update a swath of tests to push method code coverage numbers up.
Yes, method coverage is not super meaningful, but it is one metric that we can use to
ensure at least minimal coverage of this library.

Almost no product changes in here.

src/main/java/org/kohsuke/github/GitHubInteractiveObject.java

@@ -20,4 +20,8 @@ abstract class GitHubInteractiveObject {
     GitHubInteractiveObject(GitHub root) {
         this.root = root;
     }
+
+    GitHub getRoot() {
+        return root;
+    }
 }