E2E: Refactor setup method to support class inheritance in RequestUtils

[WunderBart]

This PR updates the setup static method in the RequestUtils class to use this instead of directly referencing the RequestUtils class. This change ensures that when the setup method is called on any class that extends RequestUtils, it correctly returns an instance of the calling class, thereby supporting class inheritance and extending functionality.

Before

class RequestUtils {
    static setup() {
        // Incorrectly returns an instance of the base class directly
        return new RequestUtils();
    }

    // Base class method
    baseMethod() {
        return "Base method";
    }
}

class ExtendedRequestUtils extends RequestUtils {
    // Method only available in the extended class
    extendedMethod() {
        return "Extended method";
    }
}

// Trying to use the extended class method after setup
const instance = ExtendedRequestUtils.setup();
console.log(instance instanceof ExtendedRequestUtils); // Expected true, but it's false
console.log(instance instanceof RequestUtils); // True

// Trying to access the extended method will result in an error
// console.log(instance.extendedMethod()); // TypeError: instance.extendedMethod is not a function

After

class RequestUtils {
    static setup() {
        // Dynamically returns an instance of the class it's called on
        return new this();
    }

    // Base class method
    baseMethod() {
        return "Base method";
    }
}

class ExtendedRequestUtils extends RequestUtils {
    // Method only available in the extended class
    extendedMethod() {
        return "Extended method";
    }
}

// Using the setup method properly now
const correctInstance = ExtendedRequestUtils.setup();
console.log(correctInstance instanceof ExtendedRequestUtils); // True
console.log(correctInstance instanceof RequestUtils); // True

// Accessing the extended method works as expected
console.log(correctInstance.extendedMethod()); // Outputs: "Extended method"

Testing Instructions

All the tests should pass.

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Co-authored-by: WunderBart <bartkalisz@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[Mamaduka]

Makes sense 👍