On some systems such as GitHub's Windows CI, the `write` operation
is too fast and does not cause a change in the timestamp of the
file we're reloading, hence causing the test to fail for external
reasons.
Adding a sleep does not change anything to the test functionally, but
on the contrary ensures that we are actually testing the feature.
https://stackoverflow.com/questions/19059877/python-os-path-getmtime-time-not-changing
Add two methods that are local to tests, `registerNodeDesc` and
`unregisterNodeDesc`, which handle the registration and unregistration
of `NodePlugins` from a node description. This reduces the amount of
code in tests whenever `NodePlugin` need to be instantiated prior
to their registration and so on.
