Switch Attoparsec/unicode test from QC to HUnit

This is not a quickcheck property, since it doesn't have any
variable/arbitrary inputs. So let's make it a test case, and shorten a
bit the name.

Also, sorry for not catching this in the review.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>