[tests] rename TestNode to TestP2PConn in tests

Summary:
Several test scripts define a subclass of P2PInterface called TestNode.
This commit renames those to TestP2PConn since we already have a
TestNode class in the test framework.

-BEGIN VERIFY SCRIPT-
sed -i s/TestNode/TestP2PConn/ test/functional/*py test/functional/test_framework/comptool.py
_END VERIFY SCRIPT-

Backport of Core PR12728
https://github.com/bitcoin/bitcoin/pull/12728/files

Test Plan:

make check
test_runner.py

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3747