Tests: Hard-code location of todo.sh.

As todo.sh is placed by test-lib at a fixed place, avoid the lookup through which.

tests/t0100-code-nobacktick.sh

@@ -15,7 +15,7 @@ backtick_check()
 }
 
 test_todo_session 'no old-style backtick command substitution' <<EOF
->>> backtick_check "$(which todo.sh)"
+>>> backtick_check bin/todo.sh
 EOF
 
 test_done