irule for appending a URI to the URL

Give this a try:

when HTTP_REQUEST {
    if { [HTTP::uri] equals "/" } {
        HTTP::redirect "https://[HTTP::host]/test/mypage"
    }
}

Also, it looks like you're trying to do this on HTTPS, is your virtual server SSL? If so you will require a client SSL profile and an HTTP profile to do this. If you want HTTPS terminated at the server, you will also need to use a server SSL profile.