Add node-sharing test namespace

[WilliamParker]

We currently don't have much testing around node sharing in the rules network despite a significant amount of work done in the compiler to support it. This came up recently with a fairly significant bug in issue 433. I propose adding a namespace for tests focused on testing node sharing, both on functional correctness when it is done and that we do in fact share nodes when we are intending to do so. We added a test for the particular case in issue 433 but there are likely other scenarios we should test that may or may not be related to that exact issue. This will have the added benefit of clearly establishing when we are and are not trying to share nodes, which I think is not well defined at present.

I'm currently planning to work on this in the near future but if anyone else is doing the same we should coordinate and make sure we're not duplicating work.

cc @EthanEChristian @mrrodriguez

[WilliamParker]

PR at #439