nixos/tests/systemd: Fix x-initrd-mount flakiness (#67798)

It turns out that checking for the last mount time of an ext4 file
system isn't a very reliable way to check whether the file system was
properly unmounted.

When creating that test in the first place (88530e02),
I was reluctant to inspect the file system when the VM is down and was
searching for a way to check for a clean unmount *after* the file system
was mounted again to make sure we don't need to create a 512 MB raw
image on the host.

Fortunately however, when converting from qcow2, qemu-img actually
writes a sparse file, so for most file systems (that is, file systems
supporting sparse files) this shouldn't waste a lot of disk space.

So when investigating the flakiness, I found that whenever the test is
failing, the unmount of /test-x-initrd-mount was done *before* the final
step during which systemd remounts+unmounts all the remaining file
systems.

I haven't investigated why this is the case, but the test is a
regression test for https://github.com/NixOS/nixpkgs/issues/35268

, which
actually didn't unmount the file system *at* *all*, so really all we
need to take care here is whether the unmount has happened and not
*how*.

To make sure that checking the filesystem state is enough for this, I
temporarily replaced the $machine->shutdown call with $machine->crash
and verified that the file system state is "not clean".

Signed-off-by: aszlig <aszlig@nix.build>
Fixes: https://github.com/NixOS/nixpkgs/issues/67555