[tests] Use 16 bytes key when testing Blowfish
The version of Cryptodome currently in Debian only accepts Blowfish keys
with at least 5 bytes. This is a bug (it should accept 4 as it did
before) that has since been fixed upstream:
https://github.com/Legrandin/pycryptodome/commit/93912230f3c39e09f4038cb17e301468687c6538
So we work around this issue by using a 16 bytes key instead
(which happens to be the minimum recommended length).