feat: add Cart::fake(), assertion methods, and CartFactory

Adds Cart::fake() which swaps the active driver to NullDriver and returns
a CartFactory fluent test builder. Adds assertion methods (assertContains,
assertCount, assertTotalQuantity, assertSubTotal, assertTotal,
assertConditionApplied, assertEmpty, assertNotEmpty) to Cart for use in
tests. CartFactory provides withItems(), withCondition(), and withCoupon()
as chainable helpers for seeding cart state.

src/Cart.php

@@ -5,10 +5,13 @@
 namespace Wearepixel\Cart;
 
 use Closure;
+use PHPUnit\Framework\Assert;
 use Wearepixel\Cart\Coupons\Coupon;
+use Wearepixel\Cart\Drivers\NullDriver;
 use Wearepixel\Cart\Helpers\Helpers;
 use Wearepixel\Cart\Shipping\ShippingRate;
 use Wearepixel\Cart\Tax\TaxRule;
+use Wearepixel\Cart\Testing\CartFactory;
 use Wearepixel\Cart\Validators\CartItemValidator;
 use Wearepixel\Cart\Exceptions\InvalidItemException;
 use Wearepixel\Cart\Exceptions\UnknownModelException;
@@ -49,6 +52,81 @@ public function getDriver(): CartDriver
         return $this->driver;
     }
 
+    public function fake(): CartFactory
+    {
+        $this->driver = new NullDriver($this->driver->getSessionKey());
+
+        return new CartFactory($this);
+    }
+
+    public function assertContains(int|string $id): void
+    {
+        Assert::assertTrue(
+            $this->has($id),
+            "Failed asserting that cart contains item [{$id}]."
+        );
+    }
+
+    public function assertCount(int $count): void
+    {
+        Assert::assertCount(
+            $count,
+            $this->getContent(),
+            "Failed asserting that cart has [{$count}] items."
+        );
+    }
+
+    public function assertTotalQuantity(int|float $quantity): void
+    {
+        Assert::assertEquals(
+            $quantity,
+            $this->getTotalQuantity(),
+            "Failed asserting that cart total quantity is [{$quantity}]."
+        );
+    }
+
+    public function assertSubTotal(float $amount): void
+    {
+        Assert::assertEquals(
+            $amount,
+            $this->getSubTotal(false),
+            "Failed asserting that cart subtotal is [{$amount}]."
+        );
+    }
+
+    public function assertTotal(float $amount): void
+    {
+        Assert::assertEquals(
+            $amount,
+            $this->getTotal(),
+            "Failed asserting that cart total is [{$amount}]."
+        );
+    }
+
+    public function assertConditionApplied(string $conditionName): void
+    {
+        Assert::assertNotNull(
+            $this->getCondition($conditionName),
+            "Failed asserting that condition [{$conditionName}] is applied to the cart."
+        );
+    }
+
+    public function assertEmpty(): void
+    {
+        Assert::assertTrue(
+            $this->isEmpty(),
+            'Failed asserting that the cart is empty.'
+        );
+    }
+
+    public function assertNotEmpty(): void
+    {
+        Assert::assertFalse(
+            $this->isEmpty(),
+            'Failed asserting that the cart is not empty.'
+        );
+    }
+
     public function getSessionKey(): string
     {
         return $this->driver->getSessionKey();