In order to make a useful library unit test example, the
sample_lib itself needs to perform some more interesting work.

In particular it needs at least one external function call
on which a code path decision is made, such that the example UT
code will have more than one code path to demonstrate.

Additionally, the library should have some internal "global"
structure (like real libs would) that contains its state, that
the UT can also test/query.

Both objectives can be accomplished with a small string buffer
that gets populated with a strncpy() call during init, and
printed with the "process" command.

Fix #8 
Reviewed and approved at 2019-12-18 CCB