REST: Don't call login() on the root user

We've encountered a scenario in which creating edit tags via
action.makeTag() failed after creating a bot user. The error message
indicated that the tag was attempted to be created by the bot user, even
though action.makeTag() is supposed to always create them using the root
user, see: https://github.com/wikimedia/mediawiki-tools-api-testing/blob/f19ecd38750739907d14bf4a07bb20249a27407b/lib/action_clients.js#L26

It turns out that the issue was calling root.login(), which confusingly
leads to the reference to the global root user singleton to be
overridden with whatever account is logging in. This is solved here by
simply logging in from a new anonymous user object instead of using the
root user object.

Change-Id: Ida293e426868b20afa57553bd39fa2eaa3c4e040

repo/rest-api/tests/mocha/helpers/testUsers.js

@@ -28,9 +28,10 @@ async function createUser( purpose, groups ) {
 		} )
 	);
 
-	await root.login( name, password ); // root is now the new user
+	const user = action.getAnon();
+	await user.login( name, password );
 
-	return root;
+	return user;
 }
 
 async function getOrCreateBotUser() {