[stable2407] Backport #5581 (#5603)

Backport #5581 into `stable2407` (cc @franciscoaguirre

).

The dry-run shows in `forwarded_xcms` all the messages in the queues
at the time of calling the API.
Each time the API is called, the result could be different.
You could get messages even if you dry-run something that doesn't send
a message, like a `System::remark`.

This commit fixes this by clearing the message queues before doing the
dry-run, so the only messages left are the ones the users of the API actually
care about.

Co-authored-by: Francisco Aguirre <[email protected]>