The Laravel contributing guide can be found in the Laravel documentation.
You will need to set the Stripe testing secret environment variable in a custom phpunit.xml file in order to run the Cashier tests.
Copy the default file using cp phpunit.xml.dist phpunit.xml and add the following line below the CASHIER_MODEL environment variable in your new phpunit.xml file:
<env name="STRIPE_SECRET" value="Your Stripe Secret Key"/>
Please note that due to the fact that actual API requests against Stripe are being made, these tests take a few minutes to run.