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.
For instance, on the Meshing node "outputMesh" param expression is:
value="{cache}/{nodeType}/{uid0}/mesh.{outputMeshFileTypeValue}",
So the extension here should not contains quotes.
We have the problem of spaces in file paths, choices (like colorspace),
etc.
An empty list is not send to the command line.
An empty string is send to the command line as "".
Add new unit test to ensure it follows the expected rules.
