loader: dont fail if no tests are discovered

ducktape/tests/loader.py

@@ -146,7 +146,7 @@ def load(self, symbols, excluded_test_symbols=None):
         # Sort to make sure we get a consistent order for when we create subsets
         all_test_context_list = sorted(all_test_context_list, key=attrgetter("test_id"))
         if not all_test_context_list:
-            raise LoaderException("No tests to run!")
+            self.logger.warn("No tests to run!")
         self.logger.debug("Discovered these tests: " + str(all_test_context_list))
         # Select the subset of tests.
         if self.historical_report: