Unlike "bin", "tty" actually means something on modern Linux distros,
and is used by a default udev rule, so it's more likely to exist on
distros than a group that just exists for historical compatibility
with who knows what.
"tty" exists on macOS, Debian, and Fedora (the criteria used for
choosing "bin" in f2905def ("policy tests: Use bin rather than
nogroup, wheel, or root")), but also fixes running the bus test for me
on NixOS, which has "tty" but not "bin".
Closes: https://gitlab.freedesktop.org/dbus/dbus/-/issues/514
Signed-off-by: Alyssa Ross <hi@alyssa.is>