Add test for String#test_insert.

* test/ruby/test_string.rb (test_insert): The test cases are
  written in string.c comments as a reference.  [Fix GH-1173]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53327 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,8 @@
+Sun Dec 27 11:47:46 2015  Kuniaki IGARASHI  <igaiga@gmail.com>
+
+	* test/ruby/test_string.rb (test_insert): The test cases are
+	  written in string.c comments as a reference.  [Fix GH-1173]
+
 Sun Dec 27 11:03:33 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* parse.y (show_bitstack): trace stack_type value if yydebug.

test/ruby/test_string.rb

@@ -956,6 +956,14 @@ class TestString < Test::Unit::TestCase
     assert_nil($~)
   end
 
+  def test_insert
+    assert_equal("Xabcd", S("abcd").insert(0, 'X'))
+    assert_equal("abcXd", S("abcd").insert(3, 'X'))
+    assert_equal("abcdX", S("abcd").insert(4, 'X'))
+    assert_equal("abXcd", S("abcd").insert(-3, 'X'))
+    assert_equal("abcdX", S("abcd").insert(-1, 'X'))
+  end
+
   def test_intern
     assert_equal(:koala, S("koala").intern)
     assert_not_equal(:koala, S("Koala").intern)