subscription_list test [nfc]: Split subscriptions sort test case

Subscription sort is testing too many things:
* Pinned subscriptions are coming before unpinned ones
* Pinned subscriptions are alphabetically ordered
* Unpinned subscriptions are alphabetically ordered
So what this commit does is splitting these test cases up.

Additionally some refactors are applied:
* Factor out 'checkListedStreamIdsEqual' for more concise cases
* Remove unrelated checks from the test case

Author: Khader-1

test/widgets/subscription_list_test.dart

@@ -89,18 +89,37 @@ void main() {
     check(isUnpinnedHeaderInTree()).isTrue();
   });
 
-  testWidgets('subscription sort', (tester) async {
-    await setupStreamListPage(tester, subscriptions: [
-      eg.subscription(eg.stream(streamId: 1, name: 'd'), pinToTop: true),
-      eg.subscription(eg.stream(streamId: 2, name: 'c'), pinToTop: false),
-      eg.subscription(eg.stream(streamId: 3, name: 'b'), pinToTop: true),
-      eg.subscription(eg.stream(streamId: 4, name: 'a'), pinToTop: false),
-    ]);
-    check(isPinnedHeaderInTree()).isTrue();
-    check(isUnpinnedHeaderInTree()).isTrue();
-
-    final streamListItems = tester.widgetList<SubscriptionItem>(find.byType(SubscriptionItem)).toList();
-    check(streamListItems.map((e) => e.subscription.streamId)).deepEquals([3, 1, 4, 2]);
+  group('subscription sorting', () {
+    Iterable<int> listedStreamIds(WidgetTester tester) => tester
+      .widgetList<SubscriptionItem>(find.byType(SubscriptionItem))
+      .map((e) => e.subscription.streamId);
+
+    testWidgets('pinned are shown on the top', (tester) async {
+      await setupStreamListPage(tester, subscriptions: [
+        eg.subscription(eg.stream(streamId: 1, name: 'a'), pinToTop: false),
+        eg.subscription(eg.stream(streamId: 2, name: 'b'), pinToTop: true),
+        eg.subscription(eg.stream(streamId: 3, name: 'c'), pinToTop: false),
+      ]);
+      check(listedStreamIds(tester)).deepEquals([2, 1, 3]);
+    });
+
+    testWidgets('pinned subscriptions are sorted', (tester) async {
+      await setupStreamListPage(tester, subscriptions: [
+        eg.subscription(eg.stream(streamId: 3, name: 'b'), pinToTop: true),
+        eg.subscription(eg.stream(streamId: 1, name: 'c'), pinToTop: true),
+        eg.subscription(eg.stream(streamId: 2, name: 'a'), pinToTop: true),
+      ]);
+      check(listedStreamIds(tester)).deepEquals([2, 3, 1]);
+    });
+
+    testWidgets('unpinned subscriptions are sorted', (tester) async {
+      await setupStreamListPage(tester, subscriptions: [
+        eg.subscription(eg.stream(streamId: 3, name: 'b'), pinToTop: false),
+        eg.subscription(eg.stream(streamId: 1, name: 'c'), pinToTop: false),
+        eg.subscription(eg.stream(streamId: 2, name: 'a'), pinToTop: false),
+      ]);
+      check(listedStreamIds(tester)).deepEquals([2, 3, 1]);
+    });
   });
 
   testWidgets('unread badge shows with unreads', (tester) async {