Another small optimisation in test data generation

It seems that QuickCheck will happily generate huge data structures,
if that's what one requests. For this test, where we ask for [[Char]],
it could happen that we get a huge number of chars, so let's limit
them to some (arbitrary, but low) limits. This doesn't "ruin" the
test, since the implementation of sepSplit/commaJoin is not
size-based.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>