On the contrary - I was an employee at Serverless Inc, working on the Serverless Framework for the last two years, we used this pattern extensively (and very successfully) in our open source repos.
You can even find an example here which provisions real live AWS infrastructure: https://github.com/serverless/dashboard-plugin/tree/master/i...
We used part of our enterprise SaaS product to provision temporary credentials via STS and an assumable role, and it works great. You could do the same thing with something like HC Vault.
For Lambda, S3, DynamoDB, the perpetual free tier means we've never paid to run our own tests. API Gateway isn't free (after 1 year), but it's still pennies per month. We've had several cases where tests stuck around a long time, but a billing alert and occasionally some CloudFormation stack cleanup takes care of that.
We still have offline unit tests which test business logic, but everything else runs against the cloud - even our development environments ship code straight to lambda.