test/py/tests: Move "setup_image" from test_ut.py to fs_helper.py

test/py/tests: Move "setup_image" from test_ut.py to fs_helper.py

The generic function in test_ut.py to create a disk image with partition
table can be useful outside of test_ut.py so move it to be available
more clearly.

To make this a bit more easily used library function, make use of
check_call directly rather than calling things though u_boot_utils. In
turn, to more easily handle stdin here, use the shell "printf" utility
to pass sfdisk the specification to create as we do not have an actual
file descriptor to use here.

Signed-off-by: Tom Rini <trini@konsulko.com>