adds test cases for sliver variant

test/sliver_grouped_list_test.dart

@@ -0,0 +1,68 @@
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:grouped_list/grouped_list.dart';
+import 'package:grouped_list/sliver_grouped_list.dart';
+import 'package:flutter/material.dart';
+
+final List _elements = [
+  {'name': 'John', 'group': 'Team A'},
+  {'name': 'Will', 'group': 'Team B'},
+  {'name': 'Beth', 'group': 'Team A'},
+  {'name': 'Miranda', 'group': 'Team B'},
+  {'name': 'Mike', 'group': 'Team C'},
+  {'name': 'Danny', 'group': 'Team C'},
+];
+
+void main() {
+  Widget _buildApp(List elements) {
+    return MaterialApp(
+      home: Scaffold(
+        body: Container(
+          height: 550,
+          child: CustomScrollView(
+            slivers: [
+              SliverGroupedListView(
+                groupBy: (dynamic element) => element['group'],
+                elements: _elements,
+                order: GroupedListOrder.ASC,
+                groupSeparatorBuilder: (dynamic element) =>
+                    Container(height: 50, child: Text('$element')),
+                itemBuilder: (_, dynamic element) => Container(
+                  height: 100,
+                  child: Text(element['name']),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+
+  testWidgets('finds elemets and group separators',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(_buildApp(_elements));
+
+    expect(find.text('John'), findsOneWidget);
+    expect(find.text('Team A'), findsOneWidget);
+    expect(find.text('Team B'), findsOneWidget);
+    expect(find.text('Team C'), findsWidgets);
+    expect(find.text('Danny'), findsNothing);
+
+    await tester.drag(find.byType(CustomScrollView), const Offset(0.0, -250.0));
+    await tester.pump();
+
+    expect(find.text('John'), findsNothing);
+    expect(find.text('Danny'), findsOneWidget);
+  });
+
+  testWidgets('empty list', (WidgetTester tester) async {
+    await tester.pumpWidget(_buildApp([]));
+  });
+
+  testWidgets('finds only one group separator per group',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(_buildApp(_elements));
+    expect(find.text('Team B'), findsOneWidget);
+  });
+}