RFC: Add a DEP-8 test validating that Epiphany can make connections

We run Epiphany under Xvfb to see if it will connect to a local BusyBox web server, which happens to execute a CGI script that terminates Epiphany.

I have tested this on my local machine (albeit with the version of Epiphany currently in Testing) and it works, but found that Epiphany crashes in a chroot with systemd-nspawn with repeated messages like

(epiphany:7122): epiphany-WARNING **: 11:29:23.123: Web process crashed
bwrap: loopback: Failed RTM_NEWADDR: No child processes

I'm working on debugging that.

Someone should test this in LXC or LXD to ensure that the isolation-container restriction is sufficient. Also, output to standard error is allowed to permit spurious warnings like

Gtk-Message: 12:19:13.232: Failed to load module "colorreload-gtk-module"
Gtk-Message: 12:19:13.232: Failed to load module "window-decorations-gtk-module"

which, however, do not appear to happen in a fresh virtual machine. Also, one weakness of the way this test is written is that, if Epiphany terminates on its own for some other reason than receiving a signal sent by the CGI script, then the test will still appear to pass. We can't just fall back on checking the exit status of Epiphany, because it still exits non-zero even when it receives a non-fatal signal like SIGTERM and SIGINT.

In principle though, this merge request is ready.