[Braintree] Added unit test for instant purchase PayPal token formatter

[rogyar]

Description

This PR adds unit test with different scenarios for \Magento\Braintree\Test\Unit\Model\InstantPurchase\Paypal\TokenFormatterTest class.

Fixed Issues (if relevant)

N/A

Manual testing scenarios

N/A

[magento-engcom-team]

Hi @rogyar. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

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[magento-engcom-team]

Hi @rogyar. Thank you for your contribution.
We will aim to release these changes as part of 2.2.7.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.