tests: Test for /etc/passwd instead of /etc/fstab

/etc/fstab is not guaranteed to exist on modern Linux distros

/etc/os-release won't exist on macOS but maybe that's not too important here? If someone wants to run these tests there, they can add an if statement with an appropriate alternative file.

I'm filing this merge request, because of test failures seen on Ubuntu's autopkgtest infrastructure. The armhf runners are run in a container which doesn't have /etc/fstab. I only recently started running these tests in that environment.

https://autopkgtest.ubuntu.com/packages/glibmm2.68/mantic/armhf