MM-57532: Improve timeouts for some tests (#26642)

- Bump timeouts for some cases
- Improve sleep with require/assert.EventuallyT
for better performance and reliability.

https://mattermost.atlassian.net/browse/MM-57532

```release-note
NONE
```

server/channels/api4/channel_test.go

@@ -2934,7 +2934,6 @@ func TestUpdateChannelMemberSchemeRoles(t *testing.T) {
 	_, err = SystemAdminClient.UpdateChannelMemberSchemeRoles(context.Background(), th.BasicChannel.Id, th.BasicUser.Id, s1)
 	require.NoError(t, err)
 
-	timeout := time.After(600 * time.Millisecond)
 	waiting := true
 	for waiting {
 		select {
@@ -2943,7 +2942,7 @@ func TestUpdateChannelMemberSchemeRoles(t *testing.T) {
 				require.Equal(t, model.WebsocketEventChannelMemberUpdated, event.EventType())
 				waiting = false
 			}
-		case <-timeout:
+		case <-time.After(2 * time.Second):
 			require.Fail(t, "Should have received event channel member websocket event and not timedout")
 			waiting = false
 		}
@@ -3358,7 +3357,7 @@ func TestAddChannelMemberFromThread(t *testing.T) {
 					require.EqualValues(t, float64(0), data["previous_unread_replies"])
 					require.EqualValues(t, float64(0), data["previous_unread_mentions"])
 				}
-			case <-time.After(1 * time.Second):
+			case <-time.After(2 * time.Second):
 				return
 			}
 		}