Directly test "PSS/E reimport based on exporter metadata" feature

[GabrielKS]

In #1273, I add the capability of reading a <name>_export_metadata.json file to facilitate a round trip with the PowerFlows.jl PSS/E exporter. That round trip is tested in PowerFlows.jl's test suite, but the import should also be tested in isolation in PowerSystems.