That's assuming it works correctly the first time (I haven't had that experience often, even for "trivial" code :( ). Even for "run once" functions, I still use a few tests to develop them and make sure my expectations are correct. With a good framework, setting up a handful of unit tests takes just about as much dev time as running the function in a REPL.

I'm not clear on what the scope of selenium is or how it works, but for a general web scraper, I'd identify some targets I want out of skyscanner. Here's a quick googletest butchery for "I want to make sure my function returns flights for a known good flight search."

    TEST(SkyScanner, ListFlights)
    {
        flights = MyFlightScrapingFunction(LAX, AMS, date+1 month, 1 way, 1 adult)
        EXPECT_THAT(flights, SizeIs( Gt(0) )

        EXPECT_THAT(flights, Each( AllOf( 
                                       Field(&Flight::from, Eq(LAX)), 
                                       Field(&Flight::to, Eq(AMS)),
                                       Field(&Flight::seats, Eq(1)) 
                                         )))
    }

I feel that anything simpler ("is this possible with selenium?") is a documentation moment.