Add test to enforce use of $(...) code style in todo.sh.

Pull request #26 by trajano (e24777fa2c) changed `...` to $(...), but the following commit 7900ad7e1b already added another old-style one. I think the tests can also be used for some simple enforcement of coding style, so here is a first shot at it.

Closes #46

tests/t0100-code-nobacktick.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+test_description='no old-style backtick command substitution
+
+This test checks the todo.sh script itself for occurrences
+of old-style backtick command substitution, which should be
+replaced with $(...).
+'
+. ./test-lib.sh
+
+test_todo_session 'no old-style backtick command substitution' <<EOF
+>>> sed -n -e 's/[ \t]#.*//' -e '/\d96/{=;p}' "$(which todo.sh)"
+EOF
+
+test_done