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.
Specifying the test number manually is tedious and prone to errors (as
recently proven). Since we have all the necessary data to work out the
test number, let's do it automagically.
