[keyserver] Add password sanitization tests

Summary: The sanitizeInput function will be abstracted by a general function implementing a visitor pattern (which we will also use for the server_id<->client_id conversion function) in the later diffs. This diff adds some tests so we can check that the behaviour doesn't change.

Test Plan: Run yarn jest

Reviewers: kamil, inka, tomek

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7487