filter_kanji: fix implementation, add test

lib/search/filter_kanji.dart

@@ -18,6 +18,6 @@ Future<List<String>> filterKanjiWithDbConnection(
   if (deduplicate) {
     return filteredKanji.toList();
   } else {
-    return kanji.where((k) => !filteredKanji.contains(k)).toList();
+    return kanji.where((k) => filteredKanji.contains(k)).toList();
   }
 }

test/search/filter_kanji_test.dart

@@ -0,0 +1,32 @@
+import 'package:jadb/search.dart';
+import 'package:test/test.dart';
+
+import 'setup_database_connection.dart';
+
+void main() {
+  test("Filter kanji", () async {
+    final connection = await setup_database_connection();
+
+    final result = await connection.filterKanji(
+      [
+        "a",
+        "b",
+        "c",
+        "漢",
+        "字",
+        "地",
+        "字",
+        "か",
+        "な",
+        ".",
+        "!",
+        "@",
+        ";",
+        "々",
+      ],
+      deduplicate: false,
+    );
+
+    expect(result.join(), "漢字地字");
+  });
+}