Node Config: How to prevent use of custom-environment-variables.json for Jest tests

I have a couple of Jest tests running in my NodeJS application that is configured to extract a dbConnString setting from config, using node-config.

I have a test.json file that specifies a dbConnString value (this is the one I expect the Jest tests to use). I also have a custom-environment-variables.json file that contains mappings of keys to environment variables that contain the actual connection strings to use for my dev environment.

What I'm finding is that the Jest tests are using the said environment variables mapped in custom-environment-variables.json. How can I force the Jest tests to use the dbConnString setting defined in test.json?

I have confirmed that in a test the process.env.NODE_ENV does return test.