I believe these tests may have been flaky as a result of the bug fixed
in the previous commit. A big hint is that they were all crashing with
SIGSEGV with no stack trace. I suspect that some lingering SIGIOs from
cancelations were being delivered to a thread after its `munmap` call,
which was happening because the test runner called `Io.Threaded.deinit`
to cause all of the (detached) worker threads to exit.

If this passes, I'll re-run the x86_64-linux CI jobs on this commit a
few times before merge to try and be sure there are no lingering
failures.

Resolves: https://codeberg.org/ziglang/zig/issues/30096
Resolves: https://codeberg.org/ziglang/zig/issues/30592
Resolves: https://codeberg.org/ziglang/zig/issues/30682