ws: Handle favicon.ico in test-server
ws: Handle favicon.ico in test-server

This fixes running the unit tests with `TEST_BROWSER=firefox` or
`TEST_SHOW_BROWSER`. Previously the missing favicon.ico threw off the
tests due to the 404 error.

martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit