tests: fix spurious souphttpsrc test timouts
tests: fix spurious souphttpsrc test timouts

Set GSETTINGS_BACKEND=memory, apparently there's something
about fork() and the dconf backend (or whatever else that
drags in or activates) that messes up locking and causes
timeouts due to deadlocks in g_mutex_lock(), since
everything works fine with CK_FORK=no as well.

pexip/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,stfl/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,Kurento/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good