Add Token and RegularToken fields to TestServerSetup for pre-authenticated
access. Update setupTestServer to create fresh users with valid tokens at
initialization time. Simplify createTestMediaItemID to use setup.Token.
Remove loginTestUser, loginRegularUser, loginAdminUser functions in favor
of setup.Token/setup.RegularToken. Update createTestUserOnce and
getTestUserID/getRegularUserID to be idempotent. Update all test files to
use setup.Token instead of calling login helpers.