Various testing & implementation updates:

- Added a bunch of new unit tests
- Removed test_line_buffer_fail_flush
- Updated erroneous_flush_retried
- Added helper methods to LineWriterShim for code clarity, to distinguish "self.buffer" (the BufWriter) from self.inner (the thing wrapped by the BufWriter)
- Un-expressionized write & write_all
- Added clause to bail early on Ok(0)

Author: Lucretiel