As discussed in #32973, the default value of --warn-long is roughly a minute, independent of the --long flag, and is only enabled when timing information from previous test runs is available. Our developers' guide suggests that even a "long time" test should complete in under five seconds:

https://doc.sagemath.org/html/en/developer/doctesting.html#optional-arguments

This ticket addresses two problems:

1. The timing information is often unavailable. We should still warn about long tests in that case.
2. The default of 60 CPU-equivalent seconds is much too high when --long is not in use.

Both of these issues stem from using the "wall time" to measure how long a test has run. Wall time is inherently unstable; if you're watching a movie, the tests will take longer. On a slower computer, the tests will take longer. Et cetera. The use of timing information from previous runs and the obscenely high --warn-long limit partially address that.

So in targeting our two issues, we first aim to switch the timing reports and --warn-long flag to use CPU time instead of wall time. CPU time is more reliable, since it only reports time actually spent working on the problem. (It's still sensitive to things like physical CPU time and CFLAGS; those will be addressed in #33022). With CPU time being used, the timings will be much more consistent, allowing us to lower the default value of --warn-long to 10 CPU-seconds when --long was not given.

The main difficulty is in accounting for CPU time spent in pexpect subprocesses. We don't wait() for those and they may not terminate, so the OS's usual methods of retrieving child process statistics don't work for them. At present we have a PID-based solution that works quickly on Linux/BSD, but something similar for macOS would be nice.

CC: @vbraun @tornaria

Component: doctest framework

Work Issues: cputime by PID on macOS

Author: Michael Orlitzky

Branch/Commit: u/mjo/ticket/32981 @ d7d2cff

Reviewer: https://github.com/sagemath/sage/actions/runs/1992568379

Issue created by migration from https://trac.sagemath.org/ticket/32981