Refactor socketpair tests to use utility functions for reading/writing

[hulxv]

Related to #4725

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[RalfJung]

@hulxv please do not submit half a dozen PRs at once. That feels like a DoS attack.
I'll likely have feedback for the first PR that I won't want to repeat for later PRs.

[hulxv]

@hulxv please do not submit half a dozen PRs at once. That feels like a DoS attack. I'll likely have feedback for the first PR that I won't want to repeat for later PRs.

oh sorry for that xD
I just finished the work and opened a PR for each file depends on your instruction