Fix usage of rclcpp::ok with a non-default context (#352)

* Fix usage of rclcpp::ok with a non-default context

The current implementation calls `rclcpp::ok` without any arguments,
which amounts to verifying that the global default context is valid. In
the case where a node is added to a custom context, and the global
context is not used, `rclcpp::ok` without any arguments will always
return `false` since the global context has never been initialized. To
fix it, pass to rclcpp the context that's available via the node's base
interface.

* Add a test for custom context

Signed-off-by: Christian Henkel <[email protected]>

diagnostic_updater/src/diagnostic_updater.cpp

@@ -121,7 +121,7 @@ void Updater::reset_timer()
 
 void Updater::update()
 {
-  if (rclcpp::ok()) {
+  if (rclcpp::ok(base_interface_->get_context())) {
     bool warn_nohwid = hwid_.empty();
 
     std::vector<diagnostic_msgs::msg::DiagnosticStatus> status_vec;