Factor out describe block for allowed list empty etc.

test/fun_core/basic_signup_whitelist_test.exs

@@ -5,12 +5,14 @@ defmodule FunCore.BasicSignupWhitelistTest do
     signups_allowed == email_received
   end
 
-  test "When not set, not whitelisted" do
+  describe "Not whitelisted when allowed list is" do
+    test "not set" do
       refute(mail_whitelisted_fun(nil, "joe@example.com"))
     end
 
-  test "When empty, not whitelisted" do
-    refute(mail_whitelisted_fun("", "joe@example.com"))
+    test "empty" do
+      refute(mail_whitelisted_fun("j", "joe@example.com"))
+    end
   end
 
   test "When set and mach, then whitelisted" do