checkasm: add VP9 loopfilter tests.

The randomize_buffer() implementation assures that "most of the time",
we'll do a good mix of wide16/wide8/hev/regular/no filters for complete
code coverage. However, this is not mathematically assured because that
would make the code either much more complex, or much less random.

Some fixes and improvements by Rodger Combs <rodger.combs@gmail.com>

Signed-off-by: Anton Khirnov <anton@khirnov.net>
