tests/auth/keys-t.c: Don’t ignore return value of write

tests/auth/keys-t.c: Don’t ignore return value of write

Resolves this warning:

keys-t.c: In function ‘copy’:
keys-t.c:63:6: warning: ignoring return value of ‘write’, declared with attribute warn_unused_result [-Wunused-result]
      write(out, block, len);
      ^

Change-Id: If2427f2658b428091ffba3d11643ad95f193a67d
Reviewed-on: http://gerrit.openafs.org/11957
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>