example data [nfc]: Describe when to define example actions here.

Also inline the EVENT_MUTED_USERS example to the one place it's used.
Having the data right there explicitly is nice for seeing exactly
what's happening, which makes it helpful to do that when it doesn't
get repetitious.

src/__tests__/lib/exampleData.js

@@ -25,7 +25,6 @@ import {
   ACCOUNT_SWITCH,
   LOGIN_SUCCESS,
   REALM_INIT,
-  EVENT_MUTED_USERS,
   EVENT_NEW_MESSAGE,
   MESSAGE_FETCH_START,
   MESSAGE_FETCH_COMPLETE,
@@ -673,6 +672,8 @@ export const action = deepFreeze({
     foundOldest: undefined,
     ownUserId: selfUser.user_id,
   },
+  // If a given action is only relevant to a single test file, no need to
+  // provide a generic example of it here; just define it there.
 });
 
 // Ensure every `eg.action.foo` is some well-typed action.  (We don't simply
@@ -706,11 +707,5 @@ export const eventNewMessageActionBase /* \: $Diff<EventNewMessageAction, {| mes
   // message: Message,
 };
 
-export const eventMutedUsersActionBase = {
-  type: EVENT_MUTED_USERS,
-  id: 1234,
-
-  // The details of this property are important to what a test that
-  // uses this action is testing.
-  // muted_users: [],
-};
+// If a given action is only relevant to a single test file, no need to
+// provide a generic fragment for it here; just define the test data there.

src/mute/__tests__/mutedUsersReducer-test.js

@@ -3,6 +3,7 @@ import Immutable from 'immutable';
 import deepFreeze from 'deep-freeze';
 
 import mutedUsersReducer from '../mutedUsersReducer';
+import { EVENT_MUTED_USERS } from '../../actionConstants';
 import * as eg from '../../__tests__/lib/exampleData';
 
 describe('mutedUsersReducer', () => {
@@ -36,7 +37,8 @@ describe('mutedUsersReducer', () => {
   describe('EVENT_MUTED_USERS', () => {
     test('update `muted_users` when event comes in', () => {
       const action = deepFreeze({
-        ...eg.eventMutedUsersActionBase,
+        type: EVENT_MUTED_USERS,
+        id: 1234,
         muted_users: [
           { id: eg.otherUser.user_id, timestamp: 1618822632 },
           { id: eg.thirdUser.user_id, timestamp: 1618822635 },