Makefile: Improve unittest rules
Makefile: Improve unittest rules

The executables are no longer removed when the unittest fails.

MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,libfirm/libfirm,libfirm/libfirm,libfirm/libfirm,libfirm/libfirm