Add auto-reload/watching for unit tests

[azasypkin]

Currently we use the following snippet during development:

gulp.task('watch-tests', function() {
  gulp.watch([`${TESTS_ROOT}**`], ['build-unit-tests']);
});

gulp.task('debug-unit-tests', function(cb) {
  runSequence(
    'build',
    'watch',
    'watch-tests',
    'run-unit-tests',
    cb
  );
});

We need basically the same, but a bit more polished.