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.
- change approach for test 1: screen:expect() instead of assert()
- use execute() instead of command()
- 2 new tests that check none and wrong input for :oldfiles!
Helped-by: @fwalch
Helped-by: @tarruda
Helper-by: @justinmk
