narrows tests: Fix type error missed by Flow

`initialState` wasn't a valid NarrowsState, which is

  Immutable.Map<string, $ReadOnlyArray<number>>

For what that type-checking problem is and how we plan to avoid it
in the future, see the next commit.

While we're here, make the test data a bit more interesting than the
intended value, by having the action act on a narrow that's
different from the one already in the state.

Author: chrisbobbe

src/chat/__tests__/narrowsReducer-test.js

@@ -320,23 +320,24 @@ describe('narrowsReducer', () => {
       // MESSAGE_FETCH_START applies the identity function to the
       // state (i.e., it doesn't do anything to it). Reversing that
       // effect is also done with the identity function.
-      const initialState = Immutable.Map({
-        [HOME_NARROW_STR]: {
-          older: true,
-          newer: true,
-        },
-      });
+
+      const narrow1 = pm1to1NarrowFromUser(eg.otherUser);
+      const narrow2 = pm1to1NarrowFromUser(eg.thirdUser);
+
+      // Include some other narrow to test that the reducer doesn't go mess
+      // something up there.
+      const initialState = Immutable.Map([[keyFromNarrow(narrow1), [1, 2]]]);
 
       const messageFetchStartAction = deepFreeze({
         ...eg.action.message_fetch_start,
-        narrow: HOME_NARROW,
+        narrow: narrow2,
       });
 
       const state1 = narrowsReducer(initialState, messageFetchStartAction);
 
       const messageFetchErrorAction = deepFreeze({
         type: MESSAGE_FETCH_ERROR,
-        narrow: HOME_NARROW,
+        narrow: narrow2,
         error: new Error(),
       });