ambiguous typenames used in unit tests

```
<gaston> payload_testfixture:9: error: reference to 'uint' is ambiguous
<gaston> /usr/include/sys/types.h:54: error: candidates are: typedef unsigned int uint
<gaston> ../../include/webvttxx/base:44: error:                 typedef webvtt_uint WebVTT::uint
```

I've asked gaston to see if he could reproduce the C++11-related problem (#227) on openbsd, and he's encountered a different error. If a header like sys/types is included which defines a type 'uint', then WebVTT::uint becomes ambiguous (because we're `using namespace WebVTT;`)

That kind of sucks, but we can't keep it like that, so I think we should qualify all uint/uint16/etc types in the unit tests.