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 to the <php> section in your new phpunit.xml file:
<env name="STRIPE_SECRET" value="Your Stripe Testing Secret Key"/>
Make sure to use your "testing" secret key and not your production 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.