integ-cli: fix TestImportDisplay & add FileServer

This PR fixes TestImportDisplay. HTTPS handshakes are failing erratically sometimes and that's breaking TestImportDisplay.

This PR moves the download of the file to the Dockerfile and TestImportDisplay simply imports it from a local server which serves files over HTTP.
Why do you need the `mux` if you just have one endpoint `/` ? `httptest.NewServer(handlerFunc)` should be sufficient.