Various small changes in p2p-fullblocktest.py
Summary: Formatting and making arguments explicit (whitelist is added by default unless other argument are specified, which makes it VERY confusing and got me to lose several hours of work wondering what's up).
make check ./test/functional/test_runner.py p2p-fullblocktest.py
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D1981