tests: avoid FP failure when cp fails for /proc/cpuinfo

On emulated aarch64 systems like in the qemu-based OpenBuildService
of openSUSE, cp fails to copy /proc/cpuinfo because the inode number
changes between the initial stat() call and copying the file:

  $ cp /proc/cpuinfo exp
  cp: skipping file '/proc/cpuinfo', as it was \
  replaced while being copied

* tests/cp/proc-zero-len.sh: When cp fails, check on the above
error message to skip the test.