Improve output of regression test runner

[speth]

Changes proposed in this pull request

• Restore log information printed during successful and unsuccessful runs of the regression test (test_problems) runner
• Prevent these tests from incorrectly succeeding based on reading output files from previous runs
• Return non-zero status after error from all C++ examples

If applicable, provide an example illustrating new features this pull request is introducing

Results of running scons test-cxx-LiC6-electrode:

Old per-test output (successful test):

...
clang++ -o build/samples/cxx/LiC6_electrode/LiC6_electrode.o blah blah blah
clang++ -o build/samples/cxx/LiC6_electrode/LiC6_electrode blah blah blah
* Running test 'cxx-LiC6-electrode'...

Old per-test output (missing output file):

clang++ -o build/samples/cxx/LiC6_electrode/LiC6_electrode.o blah blah blah
clang++ -o build/samples/cxx/LiC6_electrode/LiC6_electrode blah blah blah
* Running test 'cxx-LiC6-electrode'...
FAILED

New output (successful test):

clang++ -o build/samples/cxx/LiC6_electrode/LiC6_electrode.o blah blah blah
clang++ -o build/samples/cxx/LiC6_electrode/LiC6_electrode blah blah blah
* Running test 'cxx-LiC6-electrode'...
Comparing 'LiC6_electrode_blessed.csv' with 'LiC6_electrode_output.csv'
PASSED

New output (missing output file):

clang++ -o build/samples/cxx/LiC6_electrode/LiC6_electrode.o blah blah blah
clang++ -o build/samples/cxx/LiC6_electrode/LiC6_electrode blah blah blah
* Running test 'cxx-LiC6-electrode'...
Output file 'LiC6_electrode_output.csv' not found
FAILED

Checklist

• [x] The pull request includes a clear description of this code change
• [x] Commit messages have short titles and reference relevant issues
• [x] Build passes (scons build & scons test) and unit tests address code coverage
• [x] Style & formatting of contributed code follows contributing guidelines
• [x] The pull request is ready for review