Fix functional test --help raising an exception

Summary:
The test_framework.py --srcdir option referenced the default path with
%default in the help, causing argparse to expect an integer to format
the string (%d).

Test Plan:

./test/functional/test_runner.py --help

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2034