Add path_provider for elinux (#12)

Fixed #9

packages/path_provider/test/path_provider_elinux_test.dart

@@ -0,0 +1,35 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+import 'package:flutter_test/flutter_test.dart';
+import 'package:path_provider_elinux/path_provider_elinux.dart';
+import 'package:path_provider_platform_interface/path_provider_platform_interface.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  PathProviderELinux.registerWith();
+
+  test('registered instance', () {
+    expect(PathProviderPlatform.instance, isA<PathProviderELinux>());
+  });
+
+  test('getTemporaryPath', () async {
+    final PathProviderPlatform plugin = PathProviderPlatform.instance;
+    expect(await plugin.getTemporaryPath(), '/tmp');
+  });
+
+  test('getApplicationSupportPath', () async {
+    final PathProviderPlatform plugin = PathProviderPlatform.instance;
+    expect(await plugin.getApplicationSupportPath(), startsWith('/'));
+  });
+
+  test('getApplicationDocumentsPath', () async {
+    final PathProviderPlatform plugin = PathProviderPlatform.instance;
+    expect(await plugin.getApplicationDocumentsPath(), startsWith('/'));
+  });
+
+  test('getDownloadsPath', () async {
+    final PathProviderPlatform plugin = PathProviderPlatform.instance;
+    expect(await plugin.getDownloadsPath(), startsWith('/'));
+  });
+}