Replace path_provider_linux widget tests with simple unit tests (flutter#3812)

Author: stuartmorgan-g

packages/path_provider/path_provider_linux/example/test/widget_test.dart

@@ -17,4 +17,19 @@ void main() {
     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('/'));
+  });
 }