More int type fixes and tests for drivers

Just pulled in nightly and got a whole bunch of errors like this:
```
error: cannot determine a type for this expression: cannot determine the type of this integer; add a suffix to specify the type explicitly
```

Not sure, may be this just a follow-up on rust-lang/rust#6023?
Also, given it's the only field, `pub struct TestCharIO (RefCell<Option<TestCharIOData>>)` (tuple struct) is preferred