Dump all thread stacks when a unit test runs too long

When a test hangs, the job log ends with the last test that finished.
It does not show where the process waits.

Set pytest's faulthandler_timeout. When one test runs longer than 300
seconds, pytest writes the stack of every thread to stderr and lets the
test continue. The log then names the blocked test and what it waits
for.

Change-Id: I9fdb40051d6ffe6ecc5aae5eea8df1180ea215dc
Signed-off-by: Christian Schwede <cschwede@mailbox.org>
(cherry picked from commit 34a0a6443a)

tox.ini

@@ -4,6 +4,7 @@ minversion = 3.18.0
 
 [pytest]
 addopts = --verbose -p no:requests_mock -r a
+faulthandler_timeout = 300
 
 [testenv]
 usedevelop = True