sandbox: Increase default SYS_MALLOC_LEN

sandbox: Increase default SYS_MALLOC_LEN

Increase the malloc pool on sandbox in order to avoid spurious errors
such as:
___________________ test_ut[ut_dm_dm_test_video_comp_bmp32] ____________________
test/py/tests/test_ut.py:43: in test_ut
    assert output.endswith('Failures: 0')
E   AssertionError: assert False
E    +  where False = <built-in method endswith of str object at 0x7f5de85efb20>('Failures: 0')
E    +    where <built-in method endswith of str object at 0x7f5de85efb20> = 'Test: dm_test_video_comp_bmp32: video.c\r\r\nSDL renderer does not exist\r\r\ntest/dm/video.c:86, compress_frame_buff..._test_video_comp_bmp32(): 2024 == compress_frame_buffer(uts, dev): Expected 0x7e8 (2024), got 0x1 (1)\r\r\nFailures: 2'.endswith

Cc: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>