Ensure testing of `count` as result of `write`, for all backends.

[Leobouloc]

Feature Request

Is your feature request related to a problem or unsupported use case? Please describe.
It seems like there is currently no test for count (result of write) for the swift backend, in the event of an error (when 0 < count < len(data)).

(https://github.com/openfun/ralph/blob/master/tests/backends/data/test_swift.py)

Describe the solution you'd like
Add the test mentionned above for swift backend (and others if missing).

[wilbrdt]

@Leobouloc I am not sure to follow: what test would you add? Seems tested here and there