test fixnum overflow for add, sub and mul

`MRB_WORD_BOXING` is currently broken. I haven't checked, but I assume it was always the case. If this can happen, tests are needed.

Default settings and defined `MRB_WORD_BOXING`:
```ruby
x = 1
0x3fff_ffff + x  # => 1073741824.0
0x3fff_ffff + 1  # => -1073741824
0x3fff_ffff.send :+, 1  # => -1073741824
0x3fff_ffff.send :+, x  # => -1073741824
```

I have already fixed this. But this PR is only tests.