Ah, this code could be commented better I think. I did intend for the transaction to be too large for standardness rules, to ensure that we were checking for premature witness before doing the IsStandard check (that change was a recent bugfix). Perhaps just improve the comment, rather than change the test itself?

I'm not exactly clear what you were shooting for, so feel free to suggest new text?

What do you think of this:

# We'll add an unnecessary witness to this transaction that would cause 
# it to be non-standard, to test that violating policy with a witness before 
# segwit activation doesn't blind a node to a transaction.  Transactions
# rejected for having a witness before segwit activation shouldn't be added
# to the rejection cache.

And then at line 312, add a comment:

# Note that this should be rejected for the premature witness reason,
# rather than a policy check, since segwit hasn't activated yet.

sounds good, pushed