test/py: fit: Open files as binary files

The read_file() function in test_fit is used with files that are not
text files, as well as some that are. It is never used in a way that
requires it to decode text files to characters, so open all files in
binary mode such that read() doesn't attempt to decode characters for
files which are not text files.

Without this test_fit fails on python 3.x when reading an FDT in
run_fit_test() with:

  UnicodeDecodeError: 'utf-8' codec can't decode byte 0xd0 in position
                       0: invalid continuation byte

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>