tests: Fix predicate mock in simulate_narrow.

predicate is expected to return a function, not a boolean.  The
boolean true was causing _.filter to match items with a property named
"true", which is definitely not what was intended.  Matching no items
is probably also not intended, but matching every item causes the test
to fail.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

frontend_tests/node_tests/message_fetch.js

@@ -278,7 +278,7 @@ run_test('initialize', () => {
 
 function simulate_narrow() {
     const filter = {
-        predicate: function () { return true; },
+        predicate: () => () => false,
     };
 
     narrow_state.active = function () { return true; };