crypto: tcrypt - mark expected switch fall-throughs in do_test()

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-crypto@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>