Fixing usage of cmake -E compare_files instead of diff on Windows

For some reason if (WIN32 AND NOT CYGWIN) does not work anymore.

Now all TriBITS tests pass on Windows!

Build/Test Cases Summary
Enabled Packages: TriBITS
Enabled all Forward Packages
0) MPI_DEBUG => passed: passed=135,notpassed=0 (0.56 min)
1) SERIAL_RELEASE => passed: passed=135,notpassed=0 (0.53 min)