Mbed TLS supports building and running unit tests in one of two modes: “hosted” or “on target”. The on-target mode relies on Greentea, the Mbed OS test framework. The on-target mode is unmaintained (there's no CI for it and the Mbed TLS maintainers hardly ever even try to build it these days) and we're considering retiring it in Mbed TLS 2.28.
If you do use Mbed TLS's on-target unit testing, either by building target_test.function or by plugging your own file instead of target_test.function or host_test.function, please let us know and weigh in on https://github.com/ARMmbed/mbedtls/issues/4912 https://github.com/ARMmbed/mbedtls/issues/4912 .
Based on the lack of feedback from Mbed TLS users, the Mbed TLS team has concluded that there is no interest for Greentea support in Mbed TLS unit tests at this time, so we will remove it from Mbed TLS 2.28 and Mbed TLS 3.1.
We intend to keep the basic structure of the unit test framework that makes it reasonably easy to support on-target testing (in particular, the fact that you don't have to store all the bulky test data on-device at the same time). There is currently no related work on our roadmap, but we are likely to respond favorably to requests for a new on-target testing interface.