[nextest-runner] correctly dup the fd with combined output

[sunshowers]

The bug

Previously, for combined output, we were using the same file descriptor and storing it three times:

1. In .stdout
2. In .stderr
3. In state.theirs.

Stdio consumes ownership of the fd, so this effectively meant that three OwnedFd instances were present, and fds would be dropped three times.

Now in a single-threaded program this is relatively harmless -- Rust just gets EBADF for the latter two calls. But in multithreaded programs this was completely broken, with a bit of a horrific race condition on Linux. Since Tokio uses pidfds for process event handling on Linux, this caused missed wakeups in this scenario:

1. Let's say we allocated fd 20 as the theirs end provided to the child process Stdio.
2. In thread 1, we dropped the child, closing fd 20 the first time.
3. In thread 2, Tokio created a new pidfd and the kernel allocated fd 20 to this pidfd.
4. In thread 1, we continued cleaning up the child, closing fd 20 two more times.
5. The second fd 20 close caused epoll to no longer dispatch events to the pidfd.
6. After the test process exited, nextest never got a notification about that happening.

The fix

To fix the bug:

1. Don't store theirs in State -- we don't need it, and in fact had an expect(dead_code) on it.
2. Call try_clone to ensure that stdout and stderr get two separate fd numbers that refer to the same pipe.

Notes

• This wasn't originally visible because we used a single task for all threads (so there was no multithreaded race), but 5f010c7 caused each test to be in its own task. This change exposed the bug.
• This is particularly visible with pidfd because it uses the same fd namespace as regular descriptors. With more traditional SIGCHLD-based handling, the bug wouldn't be as visible.

Fixes #2295.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 78.47%. Comparing base (a1333b1) to head (4ef0253).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
nextest-runner/src/test_command/unix.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2316      +/-   ##
==========================================
- Coverage   78.49%   78.47%   -0.02%     
==========================================
  Files         105      105              
  Lines       24480    24484       +4     
==========================================
- Hits        19216    19215       -1     
- Misses       5264     5269       +5     

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