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.

test/test-lib.sh

@@ -219,10 +219,21 @@ test_fixed=0
 test_broken=0
 test_success=0
 
+declare -a _exit_functions=()
+
+at_exit_function () {
+	_exit_functions=($1 ${_exit_functions[@]/$1})
+}
+
+rm_exit_function () {
+	_exit_functions=(${_exit_functions[@]/$1})
+}
+
 _exit_common () {
 	code=$?
 	trap - EXIT
 	set +ex
+	for _fn in ${_exit_functions[@]}; do $_fn; done
 	rm -rf "$TEST_TMPDIR"
 }