[Test Coverage] Extend test coverage for BraintreeGraphQl

[TomashKhamlai]

Description:

Cover with API-functional tests:

graphql-ce/app/code/Magento/BraintreeGraphQl/Model/Resolver/CreateBraintreeClientToken.php

Line 65 in 169ac1b

$params[PaymentDataBuilder::MERCHANT_ACCOUNT_ID] = $merchantAccountId;

graphql-ce/app/code/Magento/BraintreeGraphQl/Model/BraintreeDataProvider.php

Line 43 in 1132a47

__('Required parameter "is_active_payment_token_enabler" for "braintree" is missing.')

TestCases:

Examples:

Magento\GraphQl\Customer
Magento\GraphQl\Quote\Customer

Guide:

Graphql Functional Testing