Custom program context for unit tests

Custom program context for unit tests

This improves the usability and robustness of the tests:
 - The data files in share/top/ are now always looked up from the source
   directory, without relying on any detection of the binary location.
 - It is possible to override the location of the source directory for
   some cross-compilation scenarios, which changes both the lookup
   location of share/top/ and the test input/reference files.

Change-Id: I1be5861ee594217bf27895563feb2c09fafe86c7