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/app/server_test.go

@@ -458,7 +458,7 @@ func TestSentry(t *testing.T) {
 		select {
 		case <-data:
 			require.Fail(t, "Sentry received a message, even though it's disabled!")
-		case <-time.After(time.Second):
+		case <-time.After(2 * time.Second):
 			t.Log("Sentry request didn't arrive. Good!")
 		}
 	})