Need integration test for `z_importviewkey`

[nullcopy]

Once zcash/wallet#431 merges, we need to add the corresponding integration test.

[linear]

COR-1292

[nullcopy]

For reference, here is a recently merged integration test that should be similar in shape: #95

[mnm458]

Happy to take on the integration test. I've made the fixes for zcash/wallet#431 locally and started writing the integration tests but the full end-to-end flow needs z_exportviewingkey to export a Sapling viewing key from one wallet and import it into another. So I'll pick up zcash/wallet#70 in a separate PR to unblock this.