fix role_playing_game test according to instructions

[GioeleBucci]

The exercise's instructions say: "If they player's level is below 10, their mana should be untouched."

Since the test checked against None there were unintended solutions that did not account for the player level, such as:

Some(Player(..player, health: 100, mana: option.map(player.mana, fn(_){ 100 })))

[lpil]

Hello! Isn't this unchanged and thus correct both before and after this change?

[GioeleBucci]

Hello! Isn't this unchanged and thus correct both before and after this change?

It is indeed correct, but the test uses None, which is an unfortunate value to check against since it allows defaulting to None, independently from the previous value of the "mana".
If the value to keep was, for example 7, one could ignore it and set it to None: since the test checks against that same value it would pass when it shouldn't. Changing the value the test uses to an integer would prevent such behaviour.
I don't know if I explained it in the best terms but hopefully it's understandable.