Dart fix template tests (#4015)

* Clean up samples directory before fixing tests

- removed from samples/client/petstore/dart2 :
  - purge_test.sh (doesn't seem to be used and not helpful)
  - openapi folder (is to be re-generated with more meaningful name)
- updated dart2-petstore.sh to generate client library with new name
- used updated shell script to re-generate client library
- updated CI/.drone.yml to use the new client library for tests

* Update petstore tests to use faked http client

- skipped all of the tests that hit a live endpoint
- made a fake http client that can be set to check for expected values
   and/or return a provided response
- added some files with test data recorded from live api calls
- updated the README to reflect changes to tests

* Update .drone.yml so CI will run the tests

samples/client/petstore/dart2/openapi/lib/api.dart

@@ -1,27 +0,0 @@
-library openapi.api;
-
-import 'dart:async';
-import 'dart:convert';
-import 'package:http/http.dart';
-
-part 'api_client.dart';
-part 'api_helper.dart';
-part 'api_exception.dart';
-part 'auth/authentication.dart';
-part 'auth/api_key_auth.dart';
-part 'auth/oauth.dart';
-part 'auth/http_basic_auth.dart';
-
-part 'api/pet_api.dart';
-part 'api/store_api.dart';
-part 'api/user_api.dart';
-
-part 'model/api_response.dart';
-part 'model/category.dart';
-part 'model/order.dart';
-part 'model/pet.dart';
-part 'model/tag.dart';
-part 'model/user.dart';
-
-
-ApiClient defaultApiClient = ApiClient();