test_fstring.rb: fix exception

* test/-ext-/string/test_fstring.rb (test_singleton_class): fix
  expected exception class.  [ruby-dev:49867] [Bug #12923]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56754 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/-ext-/string/test_fstring.rb

@@ -60,7 +60,7 @@ class Test_String_Fstring < Test::Unit::TestCase
   def test_singleton_class
     str = noninterned_name.force_encoding("us-ascii")
     fstr = Bug::String.fstring(str)
-    assert_raise(RuntimeError) {fstr.singleton_class}
+    assert_raise(TypeError) {fstr.singleton_class}
   end
 
   class S < String