Fix \Drupal calls should be avoided PayPlugTestService.php

Folowing #3519229 📌 Fix coding standards Active , there is still an error which would need some attention.

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  Line   tests/modules/commerce_payplug_tests/src/Services/PayPlugTestService.php  
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  94     \Drupal calls should be avoided in classes, use dependency                
         injection instead                                                         
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Injecting the HTTP client via the addArgument() method in CommercePayplugTestsServiceProvider breaks tests with the following error.

1) Drupal\Tests\commerce_payplug\Functional\OffsitePayPlugPaymentGatewayTest::testCheckoutWithOffsiteRedirectPost
ArgumentCountError: Too few arguments to function Drupal\Core\Http\HandlerStackConfigurator::__construct(), 1 passed and exactly 2 expected