simplify and speed up clean.sh in system tests
Moved here from a discussion in !1454 (merged). @ondrej has suggested we use files in system tests, similar to .gitignore files, to indicate which other files need to be removed during test cleanup.
Moved here from a discussion in !1454 (merged). @ondrej has suggested we use files in system tests, similar to .gitignore files, to indicate which other files need to be removed during test cleanup.