fix limitation in test

Test 14.10 in the test suite (of slurm 15.08.8, at least) uses

  $sinfo -tidle -h -o%n

to find idle nodes.  This only works if NodeHostname == NodeName on the
nodes.  The following should work regardless of this:

  $scontrol show hostnames \$($sinfo -tidle -h -o%N)