Instead of Luck Points...
Seems okay, just unnecessary.
It's something that would need to be tracked, and remembered, and used later. I would recommend that no more than one "held" disadvantage be allowed, and that it disappear if not used by the end of a session. Then again, that makes it just another resource for players to hoard, little different from hit points.
Some people wouldn't like that they, the player, knew that a future disadvantage was held over them. They'd find it difficult to take risks while that sword hung there, even though the character wouldn't know that a situation was any riskier than it looked. Bought-in players could probably find a way around this out-of-character influence (maybe the character has some sense of their own luck), but not everyone is that bought in.
Could players automatically fail certain tests (not trivial ones, of course) in order to get to take Advantage on a future test?
Why not just trade automatic success for automatic failure? Even with Disadvantage, the later roll could succeed. At some point, as a player attempts something, but before they roll, the GM just tells them that they fail. Or, instead of an automatic success, they get retroactive Advantage, i.e. a re-roll. They might still fail, and if they do they still have that future Disadvantage to face.
All in all, though, it seems unnecessary. Better not to roll at all, if failure is that important to avoid, especially since a GM is likely to try to find a worse situation later. If the GM isn't good at finding a worse situation later, then there's not much downside, and the choice becomes obvious.