Simplify libc-based tests with shared helpers

[RalfJung]

Many of our libc-based tests are enormously verbose: they directly interact with the libc APIs, which is cumbersome from Rust. (They also often let-bind way more than is necessary or useful.)

The goal of this issue is to make the tests more concise and therefore more readable and easier to maintain. #4724 and #4721 and good examples for the kinds of things we can do:

• use C string literals instead of manually mucking about with CString or manually null-terminating byte slice literals
• use errno_result or errno_check instead of manually checking the result of functions that report their error via errno
• use read_all_into_array instead of read/read_all, and write_all_from_slice instead of write/write_all (this won't always work!)
• use the epoll helpers in the epoll tests (I only ported a few of them)

[hulxv]

I would like to work on this :)

[hulxv]

@rustbot claim

[RalfJung]

Okay, great! Please change only 1 file (or a few small files) per PR. Also, make sure to look up the documentation of the involved libc function to understand how they report errors.