tests: New i_e2image test to validate image creation/conversion

tests: New i_e2image test to validate image creation/conversion

This commit adds new regression test called i_e2image which should
validate expected behaviour of e2image raw and qcow2 image creation
and qcow2 -> raw image conversion. You can run it with "make check" as
the rest of regression tests.

Testing is performed on three filesystem images with different block
sizes (1024, 2048, 4096). It creates raw and qcow2 images from the
original and then convert qcow2 image back to raw image. The results are
checksummed and compared with pre-prepared results. All md5sums should
stay the same and raw image created from original and qcow2 image should
be the same as well (just for the record).

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>