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.

tests/cp/proc-zero-len.sh

@@ -27,7 +27,12 @@ test -r $f || f=empty
 cat $f > out || fail=1
 
 # With coreutils-6.9, this would create a zero-length "exp" file.
-cp $f exp || fail=1
+# Skip this test on architectures like aarch64 where the inode
+# number of the file changed during the cp run.
+cp $f exp 2>err \
+  || { fail=1;
+       grep 'replaced while being copied' \
+         && skip_ "File $f is being replaced while being copied"; }
 
 # Don't simply compare contents; they might differ,
 # e.g., if CPU freq changes between cat and cp invocations.