tidy: Add a check for empty UI test files
tidy: Add a check for empty UI test files

Check for empty `.stderr` and `.stdout` files in UI test directories.
Empty files could  still pass testing for `compile-pass` tests with no output
so they can get into the repo accidentally, but they are not necessary and can
be removed.

GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust