test-lib.sh: add "atexit" functionality

test-lib.sh: add "atexit" functionality

New function at_exit_function registers given function to be called
at script termination.

Functions so registered are called in the reverse order of their
registration; no arguments are passed.

Function is called only once; re-adding with function name already
registered will remove previous registration.

New function rm_exit_function can be used to remove registration.

Modules (and possibly test-lib.sh functions) in future commits will
register such functions.