Rethink integration/e2e testing

[kung-foo]

Right now we use a simple python OPC-UA server for (very basic) integration tests. The actual invocation is pretty brittle and just spawns a background process. This testing (plus unit testing) does not give me any confidence when we make bigger changes inside the core bits.

We should switch to using the Testcontainers package for launching and running an OPC-UA server. And we should also switch (initially) to Azure's IOT OPC-UA Test server since it has a ton of built in features.

https://github.com/Azure-Samples/iot-edge-opc-plc
https://golang.testcontainers.org/

Milestone 0: Basic integration with things like ReadValue tests. Must work with Github Actions.
Milestone 1: Define how this could be used with multiple OPC-UA servers both container-based as well as "real" external instances
Milestone 2: Define what "coverage" means (i.e. maybe from the OPC-UA spec perspective) and how test cases should be written.

We are currently using Testcontainers+Azure iot-edge-opc-plc at Intelecy and it is working well.

Open questions:

• should this be in the main repo? or a separate e2e repo?
• can we approach other OPC-UA server vendors to get free licenses for testing? (e.g. Kepware, Cogent, Prosys, etc)
• how can "longer" tests be defined? i.e. we have an Aveva OPC-UA server that doesn't seem to behave correctly after an hour. how can we create a test case for that?