Address cross-module dependency in Nightmare tests

As discovered in FOLIO-1344, tests in new_proxy.js fail when ui-users is tested in isolation:

1) should add a proxy for user 1
2) should delete a sponsor of user 2

As it turns out, these tests depend ui-plugin-find-user to run, effectively making them "cross-module" tests. Tests that depend on multiple modules should live within the platform that defines all the necessary modules.

The solution here is to move these tests to platform-core. However, this may depend on timing of malc's CI work in FOLIO-1344 / FOLIO-1266 since the centralized ui-testing approach is still in play.

Also to consider: How many tests from other modules depend on ui-plugin-find-user to function?