qa: Close stdout and stderr file when node stops

Since these files are potentially deleted by the test framework for
cleanup, they should be closed first. Otherwise this will lead to errors
on Windows when the tests finish successfully.

Side note: After the patch, it is no longer possible to reopen the file
on Windows (see
https://docs.python.org/3/library/tempfile.html#tempfile.NamedTemporaryFile)
qa: Run gen_rpcauth with sys.executable

Similar to test_runner.py, the sys.executable needs to be passed down
into subprocesses to pass on native Windows. (Should have no effect on
Linux)
qa: Use files for stdout/stderr to support Windows

It seems that using PIPE is not supported on Windows. Also, it is easier
to just use the files that capture the stdout and stderr within the test
node class.

ninja check-functional

$env:PYTHONIOENCODING="utf-8"
python .\test\functional\test_runner.py --force feature_help
python .\test\functional\test_runner.py --force rpc_users