How do I use a non-Visa card when testing Google Pay using the Test Card Suite?

I'm in the process of adding a Google Pay button to a web page, as described in the Google Pay API documentation.

The button is showing on the page correctly, and I able to send encrypted payment data to my payment processor in a testing environment. For the most part, everything appears to be working as intended. We are still testing, and have not yet moved the code to production.

Following the test card suite instructions, I am able to choose from a list of credit cards, as shown in the screenshot below.

Screenshot of Google Pay's credit card selection form, showing two Visa cards, one MasterCard, one American Express card, and one Discover card

The payment information is encrypted, and I send the encrypted string to my payment processor. However, no matter which of the five test cards I choose, the decrypted credit card is always a Visa with the number 4111111111111111.

I tried checking with the payment processor, to make sure they were not using 4111111111111111 for every test transaction. They are not. They confirmed that, in all of my Google Pay tests, the number coming from Google is always the same.

Is this by design? I would like to be able to test with the other credit card types. For example, if I choose the MasterCard in the form shown above, I'd like to confirm that my payment processor sees the card as a MasterCard, and reports the last 4 digits as 4444, not 1111.

It's possible that Google always sends the same Visa number in a testing environment. However, I cannot find any documentation to support this.