tests: avoid test runner hanging on child test process join

In parallel test, the single process is spawning
a bunch of child processes running the tests,
and communicates to them. When the child process
signals that it has finished, the parent calls
child.join(). Sometimes this join never returns.
The result is a lot of defunct python processes,
and the test run just hangs.

I have seen this failure intermittently a fair bit
in a busy containerized environment, and by chance,
consistently reproduced it on a Thinkpad X280
with 8G of RAM and Ubuntu 19.04, which allowed
to diagnose it.

Type: test
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
Change-Id: If0a3110fc2d23e73d77c310d61c3ea90a2b53610

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