lava-outerr: Avoid any chance of buffering on Python side

This fixes an issue when output on feedback connections of FVP (aka
FastModels, for use with which this utility was written in the first
place) appeared only after the process terminated. The fix:

1. Runs Python interpreter with -u (ubuffered) switch.
2. Instantiates subprocess.Popen with bufsize=0 param.
3. Reads from process in small chunks, to avoid blocking
for long time.

Perhaps only one of these was needed to fix the issue at hand, but be
future-proof and apply all.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
18 jobs for lava-outerr-unbuf in 16 minutes and 49 seconds (queued for 7 seconds)
Status Name Job ID Coverage
  Test
passed dispatcher-debian-10 #140970
amd64

00:02:31

passed dispatcher-debian-11 #140972
amd64

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passed server-debian-10 #140971
amd64

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passed server-debian-11 #140973
amd64

00:05:09

 
  Analyze
passed bandit-sast #140974
amd64-dind

00:01:55

passed black #140977
amd64

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passed code_quality #140978
amd64

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passed codespell #140983
amd64

00:00:20

passed coverage #140979
amd64

00:07:23

61.02%
passed dockerfiles #140980
amd64

00:00:19

passed eslint-sast #140975
amd64-dind

00:00:30

passed pylint #140982
amd64

00:02:53

passed schemas #140981
amd64

00:00:40

passed semgrep-sast #140976
amd64-dind

00:02:04

 
  Build
passed debian/10 #140985
amd64

00:01:28

passed doc #140984
amd64

00:00:37

passed docker-amd64-dispatcher #140986
amd64-dind

00:03:45

passed docker-amd64-server #140987
amd64-dind

00:04:12