Check server output on rapper test

Thanks to Christian Ehrhardt, and Adrian Bunk for providing an
equivalent change over BTS #1001593.

This change uses a slightly different approach, but since it is already
used for the other test, it makes sense to use the same idea, to a new
dependency over netcat is not added.

Similar to the change added at 173cbfed,
add minimal sever testing to avoid the race of php web server not
launching fast enough.

debian/tests/rapper.sh

@@ -6,6 +6,15 @@ set -e
 php --server localhost:10101 --docroot debian/tests/data &
 SERVER_PID=$!
 
+set +e
+curl --fail --silent --output /dev/null http://localhost:10101/foaf.rdf
+if [ $? -ne 0 ]
+then
+    # Likely 22, the server may not be ready, give it a bit of time.
+    sleep 2
+fi
+set -e
+
 # A graphviz source file can be created.
 php debian/tests/rapper.php > sample.dot