MM-43848: Fix flaky TestUserStore/LocalCache+MySQL/UpdateUpdateAt (#20139)

Because of the fact that t.Unix() family of methods do not
contain the monotonic time, there is no guarantee that
consecutive methods will increase in time.

This is more of a best effort to double the time slept,
but in reality there is no way to control this unless
you specifically control your servers, which is hard to achieve
in a CI environment.

https://mattermost.atlassian.net/browse/MM-43848

```release-note
NONE
```

store/storetest/thread_store.go

@@ -858,7 +858,7 @@ func testVarious(t *testing.T, ss store.Store) {
 	viewThread(user1ID, dm1post1.Id)
 
 	// Add reply to a viewed thread to confirm it's unread again.
-	time.Sleep(1 * time.Millisecond)
+	time.Sleep(2 * time.Millisecond)
 	threadStoreCreateReply(t, ss, team1channel1.Id, team1channel1post2.Id, user2ID, model.GetMillis())
 
 	// Actually make team2channel1post2deleted deleted