tests: do not leave running httpd after tests exit

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
http: set the HTTP status on directory listing

Fix a HTTP response header like the following:

HTTP/1.1 0 (null)
Server: ostree-httpd libsoup/2.48.0
Date: Tue, 07 Oct 2014 11:19:22 GMT
Content-Type: text/html
Content-Length: 12533

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>