I am not aware of anyone using Tosca to test, but that doesn't mean it can't be used. I'm not familiar with it enough myself to say one way or another. Pega has built in testing tools that you might consider using. Here is a link that should provide you a number of resources.
TOSCA's regression testing tools look like they are a similar DOM (webdriver) based test framework similar to HPE QTP, Selenium and so on. For API based testing, it is likely a framework around service invocation similar to SOAPUI, Rest Assured and so on. All of these tools are compatible with Pega applications as fundamentally Pega apps run like any browser-based apps or there are service APIs in the platform or can be easily introduced for any application under test. We have a few features in the platform that can assist to make some of this testing easier such as Test ID - https://pdn.pega.com/test-id-and-tour-id-unique-identification-ui-elements/test-id-and-tour-id-unique-identification-ui