test: skip TEST-17 on ubuntu ppc64el

test: skip TEST-17 on ubuntu ppc64el

On Ubuntu CI on ppc64el, the test randomly fails when /run/udev is not
synced before checking its contents (see #22357). But /run/udev is a
tmpfs and fsync on tmpfs is noop (see `struct shmem_file_operations` in
mm/shmem.c of the kernel), hence, it is not necessary to call fsync on
/run/udev in general. This should be a testing emvironment issue (I
guess it is an issue on nested KVM on ppc64el), instead of an issue on
udev.

C.f. #22357.

Fixes #21603.