[Test Coverage] 'ApplyCouponToCart' functionality

[naydav]

Need to cover ApplyCouponToCartTest functionality with API-functional tests.
Functionality should be fixed if needed.

Query scheme: app/code/Magento/QuoteGraphQl/etc/schema.graphqls

Testcases:

\Magento\GraphQl\Quote\Guest\ApplyCouponToCartTest
- testApplyCouponToCart (should be moved and refactored from \Magento\GraphQl\Quote\CouponTest::testApplyCouponToGuestCartWithItems)
- testApplyCouponTwice (should be moved and refactored from 
 \Magento\GraphQl\Quote\CouponTest::testApplyCouponTwice)
- testApplyCouponToCartWithoutItems (should be moved and refactored from \Magento\GraphQl\Quote\CouponTest::testApplyCouponToCartWithNoItems)
- testApplyCouponToCustomerCart (should be moved and refactored from 
 \Magento\GraphQl\Quote\CouponTest::testGuestCustomerAttemptToChangeCustomerCart)
- testApplyNonExistenCouponToCart
- testApplyCouponToNonExistentCart
- testApplyExpiredCoupon
- testApplyCouponWithMissedRequiredParameters[cart_id, coupon_code] (example \Magento\GraphQl\Quote\Customer\RemoveItemFromCartTest::testUpdateWithMissedItemRequiredParameters)
- testApplyCouponWhichIsNotApplicable (products in cart are not fit to the coupon)

\Magento\GraphQl\Quote\Customer\ApplyCouponToCartTest
- testApplyCouponToCart (should be moved and refactored  from \Magento\GraphQl\Quote\CouponTest::testApplyCouponToGuestCartWithItems but for registered customer)
- testApplyCouponTwice (should be moved and refactored from 
 \Magento\GraphQl\Quote\CouponTest::testApplyCouponTwice)
- testApplyCouponToCartWithoutItems (should be moved and refactored from \Magento\GraphQl\Quote\CouponTest::testApplyCouponToCartWithNoItems)
- testApplyCouponToGuestCart
- testApplyCouponToAnotherCustomerCart
- testApplyNonExistenCouponToCart
- testApplyCouponToNonExistentCart
- testApplyExpiredCoupon
- testApplyCouponWithMissedRequiredParameters [cart_id, coupon_code] (example \Magento\GraphQl\Quote\Customer\RemoveItemFromCartTest::testUpdateWithMissedItemRequiredParameters)
- testApplyCouponWhichIsNotApplicable (products in cart are not fit to the coupon)

Check another cases in \Magento\Quote\Model\CouponManagement::set

Please, reuse current fixtures instead of creating new.

Examples you could find in
https://github.com/magento/graphql-ce/tree/2.3-develop/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Customer

https://github.com/magento/graphql-ce/tree/2.3-develop/dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Guest

[naydav]

#558