Shadow integration tests occasionally panic

A recent job failed: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/jobs/491691

This is likely runner-dependent, since no changes were made to the Shadow tests since it last passed:

$ shadow --log-level=debug --model-unblocked-syscall-latency=true snowflake-minimal.yaml > shadow.log
** Starting Shadow v3.0.0-557-g193924aa 2023-08-25--13:24:51 with GLib v2.66.8
thread 'shadow-worker' panicked at 'called `Result::unwrap()` on an `Err` value: ENOSYS', main/utility/childpid_watcher.rs:269:37
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'shadow-worker' panicked at 'called `Result::unwrap()` on an `Err` value: PoisonError { .. }', main/utility/childpid_watcher.rs:268:43
thread 'shadow-worker' panicked at 'called `Result::unwrap()` on an `Err` value: PoisonError { .. }thread '', shadow-workermain/utility/childpid_watcher.rs' panicked at ':assertion failed: self.shim_shmem_lock.borrow().is_none()268', :main/host/host.rs43:
971:9
fatal runtime error: thread local panicked on drop
thread 'shadow-worker' panicked at 'called `Result::unwrap()` on an `Err` value: PoisonError { .. }', main/utility/childpid_watcher.rs:268:43
thread 'shadow-worker' panicked at 'assertion failed: self.shim_shmem_lock.borrow().is_none()', main/host/host.rs:971:9/bin/bash: line 210: 30403 Aborted                 (core dumped) shadow --log-level=debug --model-unblocked-syscall-latency=true snowflake-minimal.yaml > shadow.log