Add a test for Dir.mktmpdir(nil) [ruby-core:66943] [Bug #10616].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48906 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/test_tmpdir.rb

@@ -30,4 +30,10 @@ class TestTmpdir < Test::Unit::TestCase
     ENV["HOME"] = home
     Dir.rmdir(dir) if dir
   end
+
+  def test_mktmpdir_nil
+    Dir.mktmpdir(nil) {|d|
+      assert_kind_of(String, d)
+    }
+  end
 end