flatpak: Add app testing manifest version

The main difference is that it builds latest GStreamer from git master and uses
runtime provided version of GTK4 (both without any patches applied).

This is done for testing purposes or finding regressions.

pkgs/flatpak/testing/gstreamer-1.0/gst-libav.json

@@ -0,0 +1,15 @@
+{
+    "name": "gst-libav",
+    "buildsystem": "meson",
+    "config-opts": [
+        "-Ddoc=disabled",
+        "-Dtests=disabled"
+    ],
+    "sources": [
+        {
+            "type": "git",
+            "url": "https://gitlab.freedesktop.org/gstreamer/gst-libav.git",
+            "branch": "master"
+        }
+    ]
+}