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).

Test Plan:

make check
./test/functional/test_runner.py p2p-fullblocktest.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1981