Refactor libc pipe tests to use utility functions for error handling and data operations

[hulxv]

Related to #4725

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[RalfJung]

Thanks a lot! This looks great.

However, there are some read calls here you did not convert because we did not have the right helpers for that. I think we should explore adding more helpers so that we don't need to do a second pass over all the tests later.

View changes since this review

[RalfJung]

We should think about a way to also cover this case without having to directly invoke read (or read_all below).

I am imagining functions like this:

/// Fill `dest` by repeatedly reading from `fd`. Error if not enough bytes could be read.
fn read_all_into_slice(fd: libc::c_int, dest: &mut [u8]) -> Result<(), libc::ssize_t>;

/// Read up to `dest.len()` many bytes from `fd`. Returns a pair of the part of `dest` that has been
/// read into, and the tail that was left untouched.
fn read_into_slice(fd: libc::c_int, dest: &mut [u8]) -> Result<(&mut [u8], &mut [u8]), libc::ssize_t>;

Can you add such functions to tests/utils/libc.rs and use them here?

(Please let me know if this is too much, then I can take over for this part.)

[RalfJung]

Suggested change

	let data = b"abcde";
	write_all_from_slice(fds[1], data).unwrap();
	write_all_from_slice(fds[1], b"abcde").unwrap();

Generally, if data is only used exactly once, then just inline it. This occurs some more times below.

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[RalfJung]

Suggested change

	use libc_utils::{errno_check, errno_result, read_all_into_array, write_all_from_slice};
	use libc_utils::*;