tests: Don't assume a fixed baud rate when identifying ttys #1409

Job	Run time
OS (apt-docker.io/amd64/ubuntu:latest)	2m 3s
OS (apt-docker.io/i386/debian:testing)	2m 11s
OS (apt-docker.io/amd64/ubuntu:devel)	2m 23s
OS (dnf-quay.io/centos/centos:stream10-development)	2m 31s
OS (dnf-registry.fedoraproject.org/fedora:rawhide)	2m 16s
OS (dnf-registry.fedoraproject.org/fedora:latest)	1m 58s
OS (alpine@gudev-docker.io/alpine)	33s
OS (apt-docker.io/amd64/ubuntu:rolling)	2m 10s
OS (alpine@gudev-alpine, EXTRA_PACKAGES=libgudev-dev)	38s
OS (apt-docker.io/amd64/debian:testing)	2m 15s
OS (nix-docker.io/nixos/nix)	1m 10s
OS (alpine-docker.io/i386/alpine)	32s
OS (alpine-docker.io/alpine)	26s
	21m 6s

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