Tests in `instrumented` directory are executed the same way as in `run` but
they have additional byte-code instrumentation performed for profiling. You
should put your tests in `instrumented` directory if you are interested in
method call counts. Examples include tests for specialization (you want to
count boxing and unboxing method calls) or high-level tests for optimizer
where you are interested if methods are successfuly inlined (so they should
not be called at runtime) or closures are eliminated (so no constructors
of closures are called).

Check `scala.tools.partest.instrumented.Instrumentation` to learn how to
use the instrumentation infrastructure.

The instrumentation itself is achieved by attaching a Java agent to the forked
VM process that injects calls to profiler. Check
`scala.tools.partest.instrumented.Instrumentation`.