Add testing of todo_completion.

Before adding any more features to todo_completion, I feel like I need test coverage, so this is a first stab at testing the completion results, via a new test function test_todo_completion.
Some basic tests showcase the capabilities.

Note: test-lib.sh now uses arrays, therefore all tests must use /bin/bash, not /bin/sh to avoid errors when sourcing test-lib. For consistency with todo.sh, we should have used Bash everywhere, anyway. Also note that t2000-multiline.sh needs some more quoting to avoid "Bash: ambiguous redirect" errors.

tests/t1600-append.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 test_description='basic append functionality