Cover low-level full mana requirement

The problem description states: `If they player's level is below 10, their mana should be None`. One of the top results did not implement this requirement due to a missing test. 

This PR adds a test verifying that low level players get their `mana` set to `None`.

exercises/concept/role-playing-game/test/role_playing_game_test.gleam

@@ -25,8 +25,8 @@ pub fn revive_a_player_that_is_alive_should_return_none_test() {
   |> should.equal(None)
 }
 
-pub fn reviving_a_low_level_player_resets_its_health_to_100_test() {
-  Player(name: None, level: 3, health: 0, mana: None)
+pub fn reviving_a_low_level_player_resets_health_and_removes_mana_test() {
+  Player(name: None, level: 3, health: 0, mana: Some(7))
   |> role_playing_game.revive
   |> should.equal(Some(Player(name: None, level: 3, health: 100, mana: None)))
 }