[PoolStatus] parse the options correctly

They used to have a field indicating how many options were present.
Now gone, presumably removed on the way upstream by a fiddler.
[thin-provisioning] Improve test_exhausting_metadata_space_causes_fail_mode

- Now checks the pool mode in the status.
- Reduce the metadata space (sometimes there was enough depending on commits)
- Remove old code that was dumping metadata due to a thin_check bug (since fixed)
- Use a variant of ProcessControl.run that allows the dd process to fail
Merge branch 'master' of github.com:jthornber/device-mapper-test-suite