Use assertion methods in `File` test

mrbgems/mruby-io/test/file.rb

 ##
-# IO Test
+# File Test
 
-assert('File', '15.2.21') do
-  File.class == Class
+assert('File TEST SETUP') do
+  MRubyIOTestUtil.io_test_setup
 end
 
-assert('File', '15.2.21.2') do
-  File.superclass == IO
+assert('File', '15.2.21') do
+  assert_equal Class, File.class
 end
 
-assert('File TEST SETUP') do
-  MRubyIOTestUtil.io_test_setup
+assert('File', '15.2.21.2') do
+  assert_equal IO, File.superclass
 end
 
 assert('File#initialize', '15.2.21.4.1') do
@@ -27,7 +27,7 @@ assert('File#path', '15.2.21.4.2') do
   assert_equal $mrbtest_io_rfname, io.path
   io.close
   assert_equal $mrbtest_io_rfname, io.path
-  io.closed?
+  assert_true io.closed?
 end
 
 assert('File.basename') do