[ddc] Alert when all hot reload tests were skipped

Add message to output when no tests were run because no tests
were found to match the filter.

Change-Id: I6dd4abd744dee9514efb02c554764e896365824d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/395049
Reviewed-by: Mark Zhou <[email protected]>
Commit-Queue: Nicholas Shahan <[email protected]>

Author: nshahan

pkg/dev_compiler/test/hot_reload_suite.dart

@@ -890,7 +890,12 @@ class HotReloadSuiteRunner {
       _print('Emitted logs to ${testResultsUri.toFilePath()} '
           'and ${testLogsUri.toFilePath()}.');
     }
-
+    if (testOutcomes.isEmpty) {
+      print('No tests ran: no sub-directories in ${allTestsUri.toFilePath()} '
+          'match the provided filter:\n'
+          '${options.testNameFilter}');
+      exit(0);
+    }
     // Report failed tests.
     var failedTests =
         testOutcomes.where((outcome) => !outcome.matchedExpectations);