Add two tests for signatures without definitions.
--- /dev/null
+module ShouldFail where
+
+sig_without_a_defn :: a -> b
+
+f :: a -> b
+f = sig_without_a_defn
--- /dev/null
+
+rnfail024.hs:3: Variable not in scope: `sig_without_a_defn'
+
+rnfail024.hs:6: Variable not in scope: `sig_without_a_defn'
+
+Compilation had errors
+
--- /dev/null
+module ShouldFail where
+
+sig_without_a_defn :: a -> b
+
+-- We don't even refer to the variable. This compiled without error
+-- in ghc-4.08.
--- /dev/null
+
+rnfail025.hs:3: Variable not in scope: `sig_without_a_defn'
+
+Compilation had errors
+