Fix mutated default argument in roundtrip tests

[stevenfontanella]

The test appends to opts which can refer to the default value of the function, causing future calls with the default value to be changed. e.g.

**>>> def f(a=[]):
...     a.append(1)
...     print(a)
...
>>> f()
[1]
>>> f()
[1, 1]

This would cause -g to be added to the arguments even when debug=False.

There are other instances that use lists as a default argument, but I left them alone for now since they never mutate it.

[tlively]

Oof, that's a big footgun.