iotests/026: Move v3-exclusive test to new file

iotests/026: Move v3-exclusive test to new file

data_file does not work with v2, and we probably want 026 to keep
working for v2 images.  Thus, open a new file for v3-exclusive error
path test cases.

Fixes: 81311255f217859413c94f2cd9cebf2684bbda94
       (“iotests/026: Test EIO on allocation in a data-file”)
Signed-off-by: Max Reitz <[email protected]>
Message-Id: <20200311140707.1243218[email protected]>
Reviewed-by: John Snow <[email protected]>
Tested-by: John Snow <[email protected]>
Signed-off-by: Max Reitz <[email protected]>