Add test for recent active-learning regression

Commit detail:
- Add a test for active learning to RunTests
- Enhance RunTests so empty stdout references are no longer required (may now remove 0-size stdout ref-files)
- Add stderr reference for active-learning regression

Note: this test currently still fails because the latest fix output has 'inf' losses, unlike the good version before the regression on May 13th.  The reference represents how the output _should_ look like.  If travis runs the test suite, it will fail.

Unrelated:
- demo/random-noise was missing one file

Thanks.