Anyway, re: the history of test screenings, here's an interesting bit of trivia from IMDB's page for Lloyd's 1921 film, "A Sailor-Made Man":
Both Lloyd and Hal Roach would haul the initial cuts of their films to theaters in the outskirts of Los Angeles for unannounced test screenings. They would carefully gauge the reactions of these audiences to individual scenes and re-cut the films accordingly. This film was unusual in that it was conceived as a 2-reel short but the 4-reel (just over 40 minutes) first cut tested so strongly with the test audience they were loathe to cut any of it. By audience default, it became his first feature-length comedy, by accident.
[...]
Voila!
.d.