test: run_seed_to_range.sh: check existance of a directory using [ -d "$dir" ]

test: run_seed_to_range.sh: check existance of a directory using [ -d "$dir" ]

sadly, sh evalutes `[ -d ]` to true. as it takes "-d" as a non-empty
string as true.

this fixes following failure
```
2016-07-12T23:22:02.839 INFO:teuthology.orchestra.run.mira084.stderr:cp:
missing destination file operand after ‘.’
2016-07-12T23:22:02.839 INFO:teuthology.orchestra.run.mira084.stderr:Try
'cp --help' for more information.
```
see
http://pulpito.ceph.com/kchai-2016-07-12_23:09:35-rados-wip-kefu-testing2---basic-mira/311334/

Signed-off-by: Kefu Chai <kchai@redhat.com>