MM-43848: Skip flaky TestUserStore/LocalCache+MySQL/UpdateUpdateAt (#20094)

https://mattermost.atlassian.net/browse/MM-43848
https://mattermost.atlassian.net/browse/MM-43871

```release-note
NONE
```

app/post_test.go

@@ -1111,6 +1111,7 @@ func TestCreatePostAsUser(t *testing.T) {
 	})
 
 	t.Run("logs warning for user not in channel", func(t *testing.T) {
+		t.Skip("MM-43871")
 		th := Setup(t).InitBasic()
 		defer th.TearDown()
 		user := th.CreateUser()

store/storetest/user_store.go

@@ -236,6 +236,7 @@ func testUserStoreUpdate(t *testing.T, ss store.Store) {
 }
 
 func testUserStoreUpdateUpdateAt(t *testing.T, ss store.Store) {
+	t.Skip("MM-43848")
 	u1 := &model.User{}
 	u1.Email = MakeEmail()
 	_, err := ss.User().Save(u1)