Perhaps existing tests in guc.sql should be merged into it?
Thanks Marko for pointing out about guc.sql.
Please find attached a patch to move DISCARD related tests from guc.sql to discard.sql. It adds an extra test for a DISCARD PLANS line, although I amn't sure on how to validate that its working.
Personally, I wouldn't call this a great patch, since most of the tests were already running, although in a generic script. The separation of DISCARD related tests to another file is arguably good for the long-term though.
