test_api_base: use file name in _calculate_chksum

test_api_base: use file name in _calculate_chksum

This is a minor clean-up which changes the '_calculate_chksum()' so that it
expects the image to be a path and does not allow for file-like objects. This
is cleaner than seeking the file-like object in this function, which changes
the object state.

This change is rather mechanical, and this is actually a preparation for the
upcoming changes, where I am going to make '_copy_image()' and some other
function generic, and then use them in a new test which I am going to add.

Change-Id: Iec472bd5f02eb9c2a1162f43fb41defe87ff3ed6
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>