test-siphash24: add a test for concatenating very short buffers

test-siphash24: add a test for concatenating very short buffers

coverity seems to think that our siphash code can read past the
end of a short buffer. Add a test which adds very short buffers
with different combinations of length to the hash. Hashing is done
twice, once with zeros following "data", and once with some other
bytes following "data". The two results are then compared to
verify that the result does not depend on bytes past the specified
data length.

(This test passes.)