This commit updates all tests to enforce the Python3 executable. This is
necessary because the `assertRegex` function we use was renamed to this
name only in Python 3.2 [1]
For reference:
s;/usr/bin/env python;/usr/bin/env python3;g
[1]: https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertRegex
The test was failing because it expected "Created task 1"
but it saw "Created task 0a436fd8-7f06-4168-a66d-bbc940498af9".
Instead, the new code looks at the description of task 1.