Issue magento/magento2#17830: Add checkout_cart_product_add_before event

 - fixed broken unit test

Author: Oleksii Korshenko

app/code/Magento/Checkout/Test/Unit/Model/CartTest.php

@@ -370,7 +370,7 @@ public function testAddProductException()
             ->method('getById')
             ->will($this->returnValue($product));
 
-        $this->eventManagerMock->expects($this->at(0))->method('dispatch')->with(
+        $this->eventManagerMock->expects($this->once())->method('dispatch')->with(
             'checkout_cart_product_add_before',
             ['info' => 4, 'product' => $product]
         );
@@ -382,10 +382,6 @@ public function testAddProductException()
             ->method('getQuote')
             ->will($this->returnValue($this->quoteMock));
 
-        $this->eventManagerMock->expects($this->never())->method('dispatch')->with(
-            'checkout_cart_product_add_after',
-            ['quote_item' => 1, 'product' => $product]
-        );
         $this->expectException(\Magento\Framework\Exception\LocalizedException::class);
         $this->cart->addProduct(4, 4);
     }