[test] combine_logs: append node stderr and stdout if it exists
[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 [[https://github.com/bitcoin/bitcoin/pull/15841 | 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

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