test/py: test_ut.py: Ensure we use bytes

In the case of some unit tests we are working with providing a fake
flash device that we have written some text strings in to.  In this case
we want to tell Python to encode things to bytes for us.

Reviewed-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Simon Glass <sjg@chromium.org> [on sandbox]
Signed-off-by: Tom Rini <trini@konsulko.com>

test/py/tests/test_ut.py

@@ -10,14 +10,14 @@ def test_ut_dm_init(u_boot_console):
 
     fn = u_boot_console.config.source_dir + '/testflash.bin'
     if not os.path.exists(fn):
-        data = 'this is a test'
-        data += '\x00' * ((4 * 1024 * 1024) - len(data))
+        data = b'this is a test'
+        data += b'\x00' * ((4 * 1024 * 1024) - len(data))
         with open(fn, 'wb') as fh:
             fh.write(data)
 
     fn = u_boot_console.config.source_dir + '/spi.bin'
     if not os.path.exists(fn):
-        data = '\x00' * (2 * 1024 * 1024)
+        data = b'\x00' * (2 * 1024 * 1024)
         with open(fn, 'wb') as fh:
             fh.write(data)