There doesn't seem to be a defined procedure for running the builtins testsuite. There is a 'test' shell script in test/builtins/Unit but it seems to be outdated and specific to darwin with hard-coded paths.
Comments in test/CMakeLists.txt suggest that builtins and BlockRunTime testsuites have not yet been ported to lit framework. Attached is a rudimentary CTest setup to run the unit tests in builtins, as a stop-gap until the test
sources can be modified for lit.
The new target to trigger test run is `check-builtins'. It is intended to stop after building tests for cross-builds, else build and execute for native builds. Hope it is useful!