field_options::struct_type::no_overrides test fails on 32-bit x86 (i686)

[decathorpe]

I'm responsible for packaging the fake and dummy crates for Fedora Linux, and I've noticed that one of the tests fails on 32-bit x86 (i686-unknown-linux-gnu target), but it might fail in the same way on other 32-bit architectures:

https://github.com/cksac/fake-rs/blob/master/fake/tests/derive_macros.rs#L166-L176

---- field_options::struct_type::no_overrides stdout ----
thread 'field_options::struct_type::no_overrides' panicked at 'assertion failed: `(left == right)`
  left: `"k5KuGzxfjPN9Hah"`,
 right: `"5KuGzxfjPN9Ha"`', tests/derive_macros.rs:175:13

I can't tell how the extra two characters (k prefix and h suffix) appear in the output, but maybe it has to do with this line:
https://github.com/cksac/fake-rs/blob/master/fake/src/impls/std/string.rs#L105

The size of usize is different on 32-bit architectures (well, per definition, it's 32 bits instead of 64), might this cause a different length to be randomly initialized?

[cksac]

updated test, should be ok now