Fix codec errors when streaming proc output

[scootermon]

While working with fancy unicode characters I noticed that every now and then my output is messed up like this:

 ╭────────────────────────────────── overview ──────────────────────────────────╮
│ ┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━┓ │
│ ┃ Test Name                        ┃ Status      ┃                Duration ┃ │
│ ┡━━━━━━━━━━━━━━━━━���━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━┩ │
│ │ smoke                            │ OK          │          0:00:00.094022 │ │

Looking into it I realized that this is caused by the incorrect handling of partial reads from the proc output.
This PR fixes the issue by making use of the built-in IncrementalDecoder - a perfect fit for this use case.

[jmaillefaud]

Fixes #438