test-lib: Avoid eating leading whitespace in expected output.

Currently, todo.sh produces no indented output, so this hasn't affected tests so far. I found this while testing one of my custom actions with the test-lib.
By unsetting IFS, the read command won't strip off leading and trailing whitespace while parsing the test session definition.

tests/test-lib.sh

@@ -582,7 +582,7 @@ test_todo_session () {
     cmd=""
     status=0
     > expect
-    while read -r line
+    while IFS= read -r line
     do
 	case $line in
 	">>> "*)