validateflow: Fail when a pad is not attached

Previously validateflow tests did not fail when the pad was not attached.

This was a limitation caused by how the Validate API worked. Before, the notify::validate-runner signal was not emitted until a monitor was attached to the override. This made impossible to listen for the runner's stopping signal.

This patch fixes the problem by setting validate-runner for all existing overrides when the runner is initialized and adding checks in validateflow to error in the case no pad was attached.