[test] combine_logs: append node stderr and stdout if it exists

Summary:
Description of issue that motivated this PR:

Currently the stdout of bitcoinds in functional tests is written to a file in self.datadir of the node. It is only read/examined after the node is shut down. To aid debugging it should (also?) be passed to the logger for visibility and timestamps.

Backport of Core PR15841
Depends on D7795

Test Plan:
Intentionaly modify a functional test to make it fail, then check that node stdout is now part of the combined logs.

ninja
../test/functional/feature_bip68_sequence.py --configfile=./test/config.ini
../test/functional/combine_logs.py -c

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7796