Check that users can’t skip steps in send test

Because we put the step in the URL, users could:
- skip ahead to a later step
- navigate to a step which doesn’t exist (ie an index greater than the
  number of placeholders)

This commit adds some checks to do the sensible thing in the unlikely
event that either of these situations occur.

app/main/views/send.py

@@ -177,6 +177,11 @@ def send_test(service_id, template_id):
 @user_has_permissions('send_texts', 'send_emails', 'send_letters')
 def send_test_step(service_id, template_id, step_index):
 
+    if 'send_test_values' not in session:
+        return redirect(url_for(
+            '.send_test', service_id=service_id, template_id=template_id
+        ))
+
     template = service_api_client.get_service_template(service_id, template_id)['data']
 
     if not session.get('send_test_letter_page_count'):
@@ -201,7 +206,12 @@ def send_test_step(service_id, template_id, step_index):
     if len(placeholders) == 0:
         return make_and_upload_csv_file(service_id, template)
 
-    current_placeholder = placeholders[step_index]
+    try:
+        current_placeholder = placeholders[step_index]
+    except IndexError:
+        return redirect(url_for(
+            '.send_test', service_id=service_id, template_id=template_id
+        ))
     optional_placeholder = (current_placeholder in optional_address_columns)
     form = get_placeholder_form_instance(
         current_placeholder,