Try to fix a fragile `File#mtime` test.

mrbgems/mruby-io/src/file.c

@@ -345,11 +345,7 @@ mrb_file_mtime(mrb_state *mrb, mrb_value self)
   fd = (int)mrb_fixnum(mrb_io_fileno(mrb, self));
   if (fstat(fd, &st) == -1)
     return mrb_false_value();
-#ifndef MRB_WITHOUT_FLOAT
-  return mrb_funcall(mrb, obj, "at", 1, mrb_float_value(mrb, st.st_mtime));
-#else
   return mrb_funcall(mrb, obj, "at", 1, mrb_fixnum_value(st.st_mtime));
-#endif
 }
 
 mrb_value

mrbgems/mruby-io/test/file.rb

@@ -73,12 +73,12 @@ assert('File#mtime') do
     skip "File#mtime require Time"
   end
   begin
-    now = Time.now.to_i
-    mt = 0
     File.open("#{$mrbtest_io_wfname}.mtime", 'w') do |f|
-      mt = f.mtime.to_i
+      assert_equal Time, f.mtime.class
+      File.open("#{$mrbtest_io_wfname}.mtime", 'r') do |f2|
+        assert_equal true, f.mtime == f2.mtime
+      end
     end
-    assert_equal true, mt >= now
   ensure
     File.delete("#{$mrbtest_io_wfname}.mtime")
   end