Add simple unit tests for binary files

[emjotde]

Description

Add a few unit tests for binary files

Checklist

• I have tested the code manually
• I have run regression tests
• I have read and followed CONTRIBUTING.md
• I have updated CHANGELOG.md

[emjotde]

To be merged before #825

[snukky]

Are these unit tests OS and CPU architecture independent?
They fail on Windows: https://github.com/marian-nmt/marian-dev/pull/826/checks?check_run_id=2028168848
If they are CPU dependent, we might need to make some expected outputs dependent on specific SIMD as we do in regression tests. We don't control what machine we get on Azure/GitHub DevOps.

To be merged before #825

This is the opposite what is in #825, could you edit one of your messages?

[emjotde]

They write and read. I will try to test it on windows to see what is breaking there. Unless you have the possibility now to take a look.

[snukky]

The issue seemed to be TemporaryFile not working properly on Windows because of missing assignment of file_, which was not related to changes from the PR. After 3eca5ed, new unit tests pass.