recipes-multimedia/gstreamer: Add gst-validate recipe

recipes-multimedia/gstreamer: Add gst-validate recipe

The gst-validate is a tool to run integration tests of Gstreamer
components [1].

This tool can be used along with gst-integration-testsuites (scenarios
and media) [2] to test Gstreamer components on the target device.

An example of test using gst-integration-testsuites:

$ gst-validate-launcher --sync # get [2] uses git and git-annex
$ gst-validate-launcher

or

$ gst-validate-launcher -nd # needs xserver-xorg-xvfb to run wo DISPLAY

[1]
https://blogs.gnome.org/tsaunier/2014/04/21/gst-validate-a-suite-of-tools-to-run-integration-tests-for-gstreamer-2/
[2] https://cgit.freedesktop.org/gstreamer/gst-integration-testsuites

Signed-off-by: Aníbal Limón <anibal.limon@linaro.org>
Signed-off-by: Ross Burton <ross.burton@intel.com>