test-lib: Allow expected output with empty lines.

An empty line is used to delimit test commands in test_todo_session. (This wasn't properly reflected in the usage comment, fixed that, too.) This prevents us from testing commands that include empty lines. (Currently, there is no such output in todo.sh, but custom add-ons may have this.)
Instead of forcing the test to filter out the empty lines, extend the syntax to allow empty lines in the expected output by escaping them with a single backslash. (When redirecting via <<EOF, the backslash itself must be escaped = doubled; better use <<'EOF'.)

tests/test-lib.sh

@@ -566,14 +566,16 @@ test_tick () {
 }
 
 # Generate and run a series of tests based on a transcript.
-# Usage: test_todo_session "description" <<EOF
+# Usage: test_todo_session "description" <<'EOF'
 # >>> command
 # output1
 # output2
+#
 # >>> command
 # === exit status
-# output3
-# output4
+# output3 with empty line (must be escaped here)
+# \
+# output5
 # EOF
 test_todo_session () {
     test "$#" = 1 ||
@@ -606,6 +608,9 @@ test_todo_session () {
 		> expect
 	    fi
 	    ;;
+	\\)
+	    echo "" >> expect
+	    ;;
 	*)
 	    echo "$line" >> expect
 	    ;;