Add unit test to verify downloading a file authenticated as UserB using a signature generated for UserA fails

Looks like it can fit nicely into this unit test in test-content.js:

`verify validation of signed urls`

We currently suspect this might be possible as the code uses `ctx.user().principalId` to get the current user.

That unit test should also have a sanity check to ensure the proper response is given before altering the parameters to get the invalid values.