test: use zero-argument super() shortcut (Python 3.0+)

Summary:

as defined in PEP 3135:

"The new syntax:

super()

is equivalent to:

super(__class__, <firstarg>)

where class is the class that the method was defined in, and <firstarg> is
the first parameter of the method (normally self for instance methods, and cls
for class methods)."

This is a backport of Core PR18585

Test Plan:
grep "super(.*, self)" test/functional/*
ninja check-functional-extended

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8988