assert_nothing_raised is not available on mruby 1.0.0.

test/io.rb

 ##
 # IO Test
 
+unless Object.respond_to? :assert_nothing_raised
+  def assert_nothing_raised(*exp)
+    ret = true
+    if $mrbtest_assert
+      $mrbtest_assert_idx += 1
+      msg = exp.last.class == String ? exp.pop : ""
+      begin
+        yield
+      rescue Exception => e
+        msg = "#{msg} exception raised."
+        diff = "      Class: <#{e.class}>\n" +
+          "    Message: #{e.message}"
+        $mrbtest_assert.push([$mrbtest_assert_idx, msg, diff])
+        ret = false
+      end
+    end
+    ret
+  end
+end
+
 assert('IO TEST SETUP') do
   MRubyIOTestUtil.io_test_setup
 end