[tests] Update start/stop node functions to be private module functions

Summary:
This commit marks the start/stop functions in util.py as private module
functions. A future PR will remove these entirely and move the
functionality directly into the BitcoinTestFramework class, but setting them as
private in this PR will prevent anyone from accidentally calling them
before that future PR is merged.

This ends the backport of core's PR10359

This was also amended to prt the test thatare specific to ABC.

Depends on D991
Closes T149

Test Plan:

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

Reviewers: #bitcoin_abc, matiu

Reviewed By: #bitcoin_abc, matiu

Subscribers: matiu, teamcity

Maniphest Tasks: T149

Differential Revision: https://reviews.bitcoinabc.org/D992