lavfi/tests: Fix 16-bit vf_blend test to avoid memory not aligned to 2 bytes

Generic C implementation of vf_blend performs reads and writes of 16-bit
elements, which requires the buffers to be aligned to at least 2-byte
boundary.

Also, the change fixes source buffer overrun caused by src_offset being
added to to test handling of misaligned buffers.

Fixes: #7226

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>