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.