Use assertion methods in FileTest tests

mrbgems/mruby-io/test/file_test.rb

@@ -67,9 +67,11 @@ assert("FileTest.size?") do
   assert_raise IOError do
     FileTest.size?(fp1)
   end
+  assert_true fp1.closed?
   assert_raise IOError do
     FileTest.size?(fp2)
   end
+  assert_true fp2.closed?
 end
 
 assert("FileTest.socket?") do
@@ -103,9 +105,11 @@ assert("FileTest.zero?") do
   assert_raise IOError do
     FileTest.zero?(fp1)
   end
+  assert_true fp1.closed?
   assert_raise IOError do
     FileTest.zero?(fp2)
   end
+  assert_true fp2.closed?
 end
 
 assert('FileTest TEST CLEANUP') do