symm/t/rijndael256: Add tests for small key sizes.

symm/t/rijndael256: Add tests for small key sizes.

Commit 388489cbb302cb86ee0fd4927243a24525dfd5ee (released in 2.4.2)
added more round constants so that we give the correct answers for
large-block Rijndael with small keys -- and this works fine for clean
builds.  Unfortunately, Catacomb's build system doesn't regenerate
recomputed tables automatically (and that would anyway be a problem for
cross builds), which means that old working trees will still be building
broken code.

Add some tests so that developers notice and hopefully rebuild the
offending tables.