Select pool based on URI and maintain with cookie

I think you can just use normal cookie persistence profile on the virtual without using an iRule for it, however you will need to use another cookie to say "use test pool". Try this;-

when HTTP_REQUEST { 
    set fAddCk 0
    if {[HTTP::cookie exists "X-testpool"]} {
        pool pl_test_http
    } elseif { [string tolower [HTTP::uri]] contains "testsite" } {
        pool pl_test_http
         Set flag to insert cookie if it doesn't already exist
        set fAddCk 1
    }
}
when HTTP_RESPONSE {
    if {$fAddCk} {
        HTTP::cookie insert name "X-testpool" value "blah"
    }
}