Yes, the PBE is for testing. But even when testing, is it too much to ask for others to be considerate? Perhaps your teammates want to test the way two late game items play together, like Shard of True Ice and Iceborn Gauntlet, and if you manage to feed, you're making it that much harder for them to do that. You also make the experience less pleasant (you don't have to win for a game to be pleasant).
If you're testing something specific, like a specific item or champion, or interaction, it's best to tell your teammates so they can help, or avoid complicating matters. Furthermore, if you don't need to test something in a PvP match, don't insist on testing it in a PvP if doing so inconveniences others.
And in the end the PBE, while the win rate on the PBE doesn't matter, using "it's just the pbe" as an excuse to do what you like makes it harder for those who are actually testing things and looking for bugs to do so. It's easiest to find bugs for the actual game when playing like it's the actual game. It's difficult to find bugs that might be situation-ally dependent on specific end-game items or champions interacting, when people feeding and playing badly because "it's just the pbe" never let you reach that point in the game.