testing/tests/boot/test_atf: fix code style

Fix these warnings:
E122 continuation line missing indentation or outdented
E302 expected 2 blank lines, found 1
F401 'os' imported but unused

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>