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.
validateflow can be used to check the buffers and events flowing through
a custom pipeline match an expectation file. This can be used to test
non-regular-playback use cases like demuxers handling adaptive streaming
fragment pushing.
This patch includes also new actions used for these cases:
`appsrc-push`, `appsrc-eos` and `flush` (plus `checkpoint`, which is
only available with validateflow).
