testsuites: validate: Add some flow integration tests for rtpsession

Hi,

I added some rtpsession integration tests to see how things would look with buffer lists in rtpsession (gst-plugins-good!149 (merged)), and to familiarize with gst-validate.

These simple tests already exposed some minor problem in rtpvrawpay, so I thought that they could be merged to spot possible future regressions.

I am marking the MR as WIP for now because the expected output depends on gst-plugins-good!177 (merged)

Thanks, Antonio