test driver: don't unlock pool after freeing it
    
     Invalid read of size 4
        at 0x945CA30: __pthread_mutex_unlock_full (in /lib64/libpthread-2.20.so)
        by 0x4F0404B: virMutexUnlock (virthread.c:94)
        by 0x4F7161B: virStoragePoolObjUnlock (storage_conf.c:2603)
        by 0x4FE0476: testStoragePoolUndefine (test_driver.c:4328)
        by 0x4FCF086: virStoragePoolUndefine (libvirt-storage.c:656)
        by 0x15A7F5: cmdPoolUndefine (virsh-pool.c:1721)
        by 0x12F48D: vshCommandRun (vsh.c:1212)
        by 0x132AA7: main (virsh.c:943)
      Address 0xfda56a0 is 16 bytes inside a block of size 104 free'd
        at 0x4C2BA6C: free (vg_replace_malloc.c:473)
        by 0x4EA5C96: virFree (viralloc.c:582)
        by 0x4F70B69: virStoragePoolObjFree (storage_conf.c:412)
        by 0x4F7167B: virStoragePoolObjRemove (storage_conf.c:437)
        by 0x4FE0468: testStoragePoolUndefine (test_driver.c:4323)
        by 0x4FCF086: virStoragePoolUndefine (libvirt-storage.c:656)
        by 0x15A7F5: cmdPoolUndefine (virsh-pool.c:1721)
        by 0x12F48D: vshCommandRun (vsh.c:1212)
        by 0x132AA7: main (virsh.c:943)