[cache tests] Improve the writethrough test a bit

It now always offset the origin device so we know the lower levels
actually change the bi_sector.

[This doesn't find an issue, but worth being there for regression
testing.]
[cache] improve the writethrough test

Hopefully it'll catch the bi_size = 0 issue now.
Merge remote branch 'ejt/master' into devel
fixed test_writethrough_ext4_uuids_match to wipe the ssd too

Now reliably reproduces Darrick's dm-devel report (ssd happened to have
the uuid on it from previous runs).

Add the :format_ssd option to with_standard_cache -- formats the first
4K of the ssd (enough to wipe the uuid and other signatures)