settings test: Check theme-setting radio buttons by checking semantics

This tests the observable behavior more directly.
RadioListTile.checked has been deprecated (#1545), so we'd like this
test to stop relying on that implementation detail.

Author: chrisbobbe

test/widgets/settings_test.dart

@@ -2,6 +2,7 @@ import 'package:checks/checks.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:legacy_checks/legacy_checks.dart';
 import 'package:zulip/model/settings.dart';
 import 'package:zulip/widgets/settings.dart';
 
@@ -39,9 +40,19 @@ void main() {
         ThemeSetting.dark => 'Dark',
       };
       for (final title in ['System', 'Light', 'Dark']) {
-        check(tester.widget<RadioListTile<ThemeSetting?>>(
-          findRadioListTileWithTitle(title)))
-            .checked.equals(title == expectedCheckedTitle);
+        final expectedIsChecked = title == expectedCheckedTitle;
+        check(tester.semantics.find(findRadioListTileWithTitle(title)))
+          // `.legacyMatcher()` wrapper just because flutter_checks's
+          //   `matchesSemantics` doesn't yet have a `hasSelectedState` param
+          .legacyMatcher(matchesSemantics(
+            label: title,
+            isFocusable: true,
+            hasEnabledState: true, isEnabled: true,
+            isInMutuallyExclusiveGroup: true,
+            hasCheckedState: true, isChecked: expectedIsChecked,
+            hasSelectedState: true,
+            hasTapAction: true,
+            hasFocusAction: true));
       }
       check(testBinding.globalStore)
         .settings.themeSetting.equals(expectedThemeSetting);