Skip tmpfs-sensitive tests if fstype cannot be determined

Skip tmpfs-sensitive tests if fstype cannot be determined

In certain isolation environments (f.e. Debian's sbuild+unshare),
`/proc/mounts` does not have an entry for the directory we're building in.

Then, findmnt as used in the tests returns nothing. As the build directory
might very well be on tmpfs (and often is, in Debian), disable the two tests
known to fail on tmpfs.

In yet another scenario, if ran in a Docker/podman container, the fstype
will read "overlay", which in the end can also be tmpfs, and we see problems
there, too.

This was noticed because torvalds/linux@e88e0d366f9cfbb810b0c8509dc5d130d5a53e02
changed the behaviour of the underlying syscalls on tmpfs in Linux 6.6.

Closes: #3266
Debian bug #1086706

Signed-off-by: Chris Hofstaedtler <zeha@debian.org>