REFORM-1144 Restructure checksum comparison code (#59)

* Create a new Compare class in TestChecksumCompare.(cc|hh) that
  contains the code from the function Test::compare_checksums()
  (previously in ReForm Preprocess) and Test::compare_reports()
  (previously in TestResultAnalysis.(cc|hh)).

  Checksum objects reside in the binary that implements the tested
  algorithms (A), which may not be the same binary that runs the
  comparisons (B). In order to deal with this, we create an instance
  of the Compare class for each copy of Base contained within the
  final binary. The Compare object from A can be provided to B to
  perform the comparisons.

* Create NullOutputStream and a global instance Base::null_os. This
  ignores any data streamed to it.

Co-authored-by: Martin Marinov <martin.marinov@autodesk.com>