Adopt the Food class and its tests

Refs #11
Task: MIL-002#3

src/snake_game/food.py

@@ -0,0 +1,49 @@
+"""! @file
+@brief The food: a small dot that the snake eats.
+
+The class follows the day-21 lecture on inheritance: `Food` is a `Turtle` that
+sets itself up and moves to a random place. Because it inherits from `Turtle`,
+this module needs `turtle` (and so `tkinter`) when it is imported; the game
+imports it only when it starts, and the tests install a fake `turtle` module
+first.
+"""
+
+import random
+from turtle import Turtle
+
+from snake_game.constants import (
+    FOOD_COLOR,
+    FOOD_SHAPE,
+    FOOD_SIZE,
+    FOOD_SPEED,
+    WALL_LIMIT,
+)
+
+
+def random_coordinate() -> int:
+    """! @brief Pick a random x or y inside the walls.
+
+    All the randomness of the food comes from this one function, so that a test can
+    replace it.
+
+    @return A whole number from `-WALL_LIMIT` to `WALL_LIMIT`.
+    """
+    return random.randint(-WALL_LIMIT, WALL_LIMIT)
+
+
+class Food(Turtle):
+    """! @brief The food, a blue circle at a random place inside the walls."""
+
+    def __init__(self) -> None:
+        """! @brief Set the food up and put it at a random place."""
+        super().__init__()
+        self.shape(FOOD_SHAPE)
+        self.penup()
+        self.shapesize(stretch_wid=FOOD_SIZE, stretch_len=FOOD_SIZE)
+        self.color(FOOD_COLOR)
+        self.speed(FOOD_SPEED)
+        self.refresh()
+
+    def refresh(self) -> None:
+        """! @brief Move the food to a new random place inside the walls."""
+        self.goto(random_coordinate(), random_coordinate())