I am lately a fan of test reports written in a way @robertday describes, brief and intended for specific non-tester audiences.
My test plan on the other hand is a resourcing plan, and I can frankly find no good reason to put the 2 into one document, that and the fact that the plan usually has to be reviewed before the execution that goes into the report starts, keeping the document “live” along the entire process feels like unfinished work.