Consider value of system tests

[MatthewHambley]

Currently the system tests are really just testing the top level program as all the other functionality is well covered by unit tests. Their value should be assessed.

In particular it might be better to replace them with something more like acceptance tests which build real-world example code bases that we care about.

Resource usage may be an issue there.

[t00sa]

This is waiting for the lfric core repository to become public, then it can be used as a target for useful testing. The goal is to have a set of acceptance tests that can be run against a set of real repositories to convince us that we haven't lost our ability to build them.