Fix reporting of failed C++ tests (FWNX-1050)

Failing tests are determined by setting the property FailedSuites
which the calls of the Unitpp task didn't add before. We don't
want to continue on error when calling the Unitpp task - an error
is only reported when the test fixture crashes.

Change-Id: I1bf0a3e4d2839f432e42fe23d3c7df13f40e3b46