We check all pairs of possible formats and for every pair write some
data at file 0 block, check that it is in source format; then change
default format, write another data, check that file.blktab[0] changed
its type.

I.e. with this test we verify that the we can read data in old format
and change it incrementally to new. Or the other way, if data is in new
format, and we decide to migrate to old one, it also works.

/cc @Tyagov, @klaus