Add automated tests for partitioning

[robertbartel]

As discussed in noaa-owp/nwm-qa#149 and the related #904, there currently are some issues with partitioning and/or partition config file generation. Additional automated tests for proper handling of various scenarios will assist with uncovering any problems contributing to that issue. They will also be helpful more generally in maintaining the quality of the partitioning code, so we should try to add automated tests to cover a variety of different possible scenarios to ensure valid partitioning and file generation.

[robertbartel]

One specific thing to ensure is tested is whether the created partition file always correctly adds remote connections for any nexus appearing in two different partitions (see #904).

[robertbartel]

We likely need to revisit #315 and #317.