regtest: Do not buffer stderr output
Some buggy documents can produce a huge stderr output because of parsing errors or whatever. We could give a file directly to Popen to write the stderr file, but we only want to create the file when there's output, because it's what we use to know whether the command produced output or not. So, instead of buffering the whole output and then write it to the file, now we read from the pipe while the command is running, writing the output in chunks to the file. This improves a lot the memory consumption when running some tests.