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/t6000-completion.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+#
+
+test_description='Bash completion functionality
+
+This test checks basic todo_completion of actions and options
+'
+. ./test-lib.sh
+
+readonly ACTIONS='add a addto addm append app archive command del rm depri dp do help list ls listall lsa listcon lsc listfile lf listpri lsp listproj lsprj move mv prepend prep pri p replace report shorthelp'
+readonly OPTIONS='-@ -@@ -+ -++ -d -f -h -p -P -PP -a -n -t -v -vv -V -x'
+
+test_todo_completion 'all arguments' 'todo.sh ' "$ACTIONS $OPTIONS"
+test_todo_completion 'arguments beginning with a' 'todo.sh a' 'add a addto addm append app archive'
+test_todo_completion 'all options' 'todo.sh -' "$OPTIONS"
+test_todo_completion 'all actions after command action' 'todo.sh command ' "$ACTIONS"
+
+test_done