api-testing/REST/Update.js: Use named user for edits
Why:
- The tests in this file aren't specifically about anonymous editing, and using a named user for the test edits makes it easier to support this test in both a temp accounts and non-temp-accounts context
What:
- Switch the actor used in making edits for the fixtures in the tests to be a named user
Bug: T365647
Change-Id: Ic3cc846892b03a9a550f39a92686ae904cfa2c82