tests: Fix souphttpsrc tests without CK_FORK=no
tests: Fix souphttpsrc tests without CK_FORK=no

It seems that the forked processes all attempt to handle the listening
socket from the server, and only one has to shutdown the socket to break
the server completely.

Create a new server inside each test to avoid this.

https://bugzilla.gnome.org/show_bug.cgi?id=772656

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