It is otherwise impossible to determine which test failed sanitizer/valgrind
check. test/functional/helpers.lua module return was changed so that tests which
do not provide after_each function to get new check will automatically fail.
os.remove() wasn't removing the temporary swap directory which leads to
problems when the test is run a second time.
That's also the reason why the CI never caught this.
os.remove() got replaced by helpers.rmdir().
