tests: Make test writing simpler and add two new tests.

Add a test_todo_session function to test-lib.sh that enables
easy recording of input and output from todo.sh (including
annotation of exit status for testing error cases).

Begin to port and re-factor testsuite.txt into smaller
test cases, starting with basic add list functionality
and replace functionality.

Thanks to Philippe Teuwen <phil@teuwen.org> for feedback
and improvements.

Signed-off-by: Emil Sit <sit@emilsit.net>

tests/t1100-replace.sh

@@ -0,0 +1,71 @@
+#!/bin/sh
+
+test_description='basic replace functionality
+
+Ensure we can replace items successfully.
+'
+. ./test-lib.sh
+
+#
+# Set up the basic todo.txt
+#
+todo.sh add notice the daisies > /dev/null
+
+test_todo_session 'replace usage' <<EOF
+>>> todo.sh replace adf asdfa
+=== 1
+usage: $(which todo.sh) replace ITEM# "UPDATED ITEM"
+EOF
+
+test_todo_session 'basic replace' <<EOF
+>>> todo.sh replace 1 "smell the cows"
+1: notice the daisies
+replaced with
+1: smell the cows
+
+>>> todo.sh list
+1 smell the cows
+--
+TODO: 1 of 1 tasks shown from $HOME/todo.txt
+
+>>> todo.sh replace 1 smell the roses
+1: smell the cows
+replaced with
+1: smell the roses
+
+>>> todo.sh list
+1 smell the roses
+--
+TODO: 1 of 1 tasks shown from $HOME/todo.txt
+EOF
+
+cat > todo.txt <<EOF
+smell the cows
+grow some corn
+thrash some hay
+chase the chickens
+EOF
+test_todo_session 'replace in multi-item file' <<EOF
+>>> todo.sh replace 1 smell the cheese
+1: smell the cows
+replaced with
+1: smell the cheese
+
+>>> todo.sh replace 3 jump on hay
+3: thrash some hay
+replaced with
+3: jump on hay
+
+>>> todo.sh replace 4 collect the eggs
+4: chase the chickens
+replaced with
+4: collect the eggs
+EOF
+
+test_todo_session 'replace error' << EOF
+>>> todo.sh replace 10 "hej!"
+=== 1
+10: No such todo.
+EOF
+
+test_done