This library provides some simple APIs and glue code for test code that is meant to run exclusively as part of a standalone test executable launched directly by userboot. Such tests can't use any normal system services, since there are no other services running at all. Instead, they can directly use the privileged resources and VMOs provided by the kernel. The lib/standalone-test/standalone.h header provides simple C++ functions in the standalone namespace for getting these handles.
Standalone tests like this need to take care to minimize their library dependencies, avoiding anything like fdio that interacts with other system services. Instead, linking in the standalone-test provides some simple replacements for functions like write that send any stdout/stderr text lines as kernel debuglog messages.