test: `ReaderParser` has no unit-tests.

### Request

We've been running into bugs with `ReaderParser`. The unit-tests currently focus on `BufferParser`. While these two classes share a lot of functionality, the `ReaderParser` has the unique task of handling buffer wrap around.

### Possible implementation

Add unit-tests to test:
- The wrap around case.
- Reusing a Token (also testing the wrap around case).