tests: Correct timing of python tests with python3 < 3.7

tests: Correct timing of python tests with python3 < 3.7

`time.monotonic_ns()` was introduced in python 3.7. Prior to that, the
other available monotonic time function was `time.monotonic()` which was
itself introduced in python 3.3.

For python3 < 3.3, the automatic timing of TAP tests is disabled.

The use of underscores for readable integers was also only introduced in
python 3.6.

Change-Id: Ibf85669c4d108347097d2cea7ab5d28cde9d0cc6
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>