tests: net: http_server: Add tests for the HTTP server

Tests for HTTP server support.

Signed-off-by: Emna Rekik <emna.rekik007@gmail.com>
Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>