MultiThreadedExecutionTestCase fails nondeterministically on Travis

[jacquev6]

https://travis-ci.org/jacquev6/ActionTree/jobs/326011923#L731
https://travis-ci.org/jacquev6/ActionTree/jobs/314271763#L733
https://travis-ci.org/jacquev6/ActionTree/jobs/313910394#L733
https://travis-ci.org/jacquev6/ActionTree/jobs/312889258#L733
https://travis-ci.org/jacquev6/ActionTree/jobs/310802825#L756

All tests in MultiThreadedExecutionTestCase seem to fail from time to time. This seems to happen only on Python 3. I was not able to reproduce it on my desktop computer.

[jacquev6]

Testing multi-threaded code using time.sleep to synchronize threads is not robust. This seemed reliable enough when I wrote the test code, but this proves it's not.

I'll first make the delay 10 times longer on Travis to verify that the issue is indeed in the testing framework and not in the library. Then I'll work on a more robust testing strategy.

[jacquev6]

"Then I'll work on a more robust testing strategy": done.