Move end-to-end tests into their own directory (#2322)

These are the TODO. These are the slow ones to migrate to be unit tests, and
speed up.

Author: srawlins

test/unit/comment_processable_test.dart renamed to test/comment_processable_test.dart

@@ -12,15 +12,15 @@ import 'package:dartdoc/dartdoc.dart';
 import 'package:path/path.dart' as path;
 import 'package:test/test.dart';
 
-import '../tool/subprocess_launcher.dart';
-import 'src/utils.dart';
+import '../../tool/subprocess_launcher.dart';
+import '../src/utils.dart';
 
 Uri get _currentFileUri =>
     (reflect(main) as ClosureMirror).function.location.sourceUri;
 String get _testPackagePath =>
-    path.fromUri(_currentFileUri.resolve('../testing/test_package'));
-String get _testPackageFlutterPluginPath => path
-    .fromUri(_currentFileUri.resolve('../testing/test_package_flutter_plugin'));
+    path.fromUri(_currentFileUri.resolve('../../testing/test_package'));
+String get _testPackageFlutterPluginPath => path.fromUri(
+    _currentFileUri.resolve('../../testing/test_package_flutter_plugin'));
 
 void main() {
   group('Invoking command-line dartdoc', () {
@@ -210,8 +210,8 @@ void main() {
     }, timeout: Timeout.factor(2));
 
     test('--footer-text excludes version', () async {
-      var _testPackagePath = path
-          .fromUri(_currentFileUri.resolve('../testing/test_package_options'));
+      var _testPackagePath = path.fromUri(
+          _currentFileUri.resolve('../../testing/test_package_options'));
 
       var args = <String>[dartdocPath, '--output', tempDir.path];
 

test/dartdoc_integration_test.dart renamed to test/end2end/dartdoc_integration_test.dart

@@ -17,7 +17,7 @@ import 'package:dartdoc/src/warnings.dart';
 import 'package:path/path.dart' as path;
 import 'package:test/test.dart';
 
-import 'src/utils.dart';
+import '../src/utils.dart';
 
 final _resourceProvider = pubPackageMetaProvider.resourceProvider;
 final _pathContext = _resourceProvider.pathContext;

test/dartdoc_test.dart renamed to test/end2end/dartdoc_test.dart

@@ -16,7 +16,7 @@ import 'package:dartdoc/src/warnings.dart';
 import 'package:path/path.dart' as path;
 import 'package:test/test.dart';
 
-import 'src/utils.dart' as utils;
+import '../src/utils.dart' as utils;
 
 // Init a generator without a GeneratorContext and with the default file writer.
 Future<Generator> _initGeneratorForTest() async {

test/html_generator_test.dart renamed to test/end2end/html_generator_test.dart

@@ -19,7 +19,7 @@ import 'package:dartdoc/src/special_elements.dart';
 import 'package:pub_semver/pub_semver.dart';
 import 'package:test/test.dart';
 
-import 'src/utils.dart' as utils;
+import '../src/utils.dart' as utils;
 
 final String _platformVersionString = Platform.version.split(' ').first;
 final Version _platformVersion = Version.parse(_platformVersionString);

test/model_special_cases_test.dart renamed to test/end2end/model_special_cases_test.dart

@@ -19,7 +19,7 @@ import 'package:dartdoc/src/special_elements.dart';
 import 'package:dartdoc/src/warnings.dart';
 import 'package:test/test.dart';
 
-import 'src/utils.dart' as utils;
+import '../src/utils.dart' as utils;
 
 final _testPackageGraphMemo = AsyncMemoizer<PackageGraph>();
 Future<PackageGraph> get _testPackageGraph =>