tests: Adjust graph test.

This is a followup to 301a4249064227bc4883e4e5b638e85a65925ba0, which
added a dependency to sed.

* tests/graph.scm ("node-edges"): Use 'hello', not 'sed', as the example
of a package without any dependency.

tests/graph.scm

@@ -293,7 +293,7 @@ edges."
   (run-with-store %store
     (let ((packages (fold-packages cons '())))
       (mlet %store-monad ((edges (node-edges %package-node-type packages)))
-        (return (and (null? (edges sed))
+        (return (and (null? (edges hello))
                      (lset= eq?
                             (edges guile-2.0)
                             (match (package-direct-inputs guile-2.0)