added a local backend for the tests

The tests currently fail in netword-restricted build environments, it is
solved by starting a local backend for each test suites.

The backend is built on top of python's BaseHTTPServer, which is already
present at build time. It simply answers empty HTTP responses (204) to
any GET, HEAD or POST requests.

Conflicts:
	tests/badvarnishd.sh
	tests/util.sh