Extend timeouts to resolve flakiness on macs, windows bots (#2177)

Author: jcollins-g

test/dartdoc_test.dart

@@ -435,5 +435,5 @@ void main() {
             startsWith('Unsupported output format'));
       }
     });
-  }, timeout: Timeout.factor(8));
+  }, timeout: Timeout.factor(12));
 }

test/model_special_cases_test.dart

@@ -586,5 +586,5 @@ void main() {
               'parameters. FormatException: Could not find an option named "name".'),
           isTrue);
     });
-  });
+  }, timeout: Timeout.factor(2));
 }