scripted-diff: Use UniValue.pushKV instead of push_back(Pair())

Summary:
-BEGIN VERIFY SCRIPT-
git grep -l "push_back(Pair" | xargs sed -i "s/push_back(Pair(\(.*\)));/pushKV(\1);/g"
-END VERIFY SCRIPT-

Depends on D1989

Test Plan:

make check && ./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1990