[qa] fix abc-p2p-compactblocks test to use add_argument

Summary: As per title, fixes issue that was introduced with D2010

Test Plan:

make && ./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2015