MM-43871: Fix flaky TestCreatePostAsUser (#20484)

We need to flush the logger before checking
the log results. Without that, sometimes
it would get logged after checking the line,
as can be seen in the JIRA ticket.

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

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/post_test.go

@@ -1108,7 +1108,6 @@ 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()
@@ -1123,6 +1122,8 @@ func TestCreatePostAsUser(t *testing.T) {
 		_, appErr := th.App.CreatePostAsUser(th.Context, post, "", true)
 		require.Nil(t, appErr)
 
+		require.NoError(t, th.TestLogger.Flush())
+
 		testlib.AssertLog(t, th.LogBuffer, mlog.LvlWarn.Name, "Failed to get membership")
 	})
 
@@ -1146,6 +1147,8 @@ func TestCreatePostAsUser(t *testing.T) {
 		_, appErr = th.App.CreatePostAsUser(th.Context, post, "", true)
 		require.Nil(t, appErr)
 
+		require.NoError(t, th.TestLogger.Flush())
+
 		testlib.AssertNoLog(t, th.LogBuffer, mlog.LvlWarn.Name, "Failed to get membership")
 	})