Prefer function output over input

When learning about the behavior of a function, in general the learning
experience is better when provided with the input and asked for the
output.

lib/koans/02_strings.ex

@@ -32,10 +32,10 @@ defmodule Strings do
   end
 
   koan "Repetition is the mother of learning" do
-    assert "StringStringString" == String.duplicate(___, 3)
+    assert String.duplicate("String", 3) == ___
   end
 
   koan "Strings can be louder when necessary" do
-    assert "LISTEN" == String.upcase(___)
+    assert String.upcase("listen") == ___
   end
 end

test/koans/strings_koan_test.exs

@@ -11,8 +11,8 @@ defmodule StringTests do
       "incredible",
       "banana",
       "banana",
-      "String",
-      "listen",
+      "StringStringString",
+      "LISTEN",
     ]
 
     test_all(Strings, answers)