Support for activity/test tracing.

[LuDubies]

Additionally to tracing source code requirements, a sensible method of tracing tests has to be defined and implemented.

lobster-python as an example allows the tracing of pyunit and unittest tests in python.

As Rust supports nested tests in source code comments and tests in the same source file, impementation of this feature could prove complex.
But this easily accessible way of testing in Rust also presents itself as a great starting point for test tracing, as its use is likely widespread in many applications.