Tests: Add a basic UTF-8 to UTF-8 LibTextCodec test

Tests/LibTextCodec/TestTextDecoders.cpp

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Vector.h>
+#include <LibTest/TestCase.h>
+#include <LibTextCodec/Decoder.h>
+
+TEST_CASE(test_utf8_decode)
+{
+    auto decoder = TextCodec::UTF8Decoder();
+    // Bytes for U+1F600 GRINNING FACE
+    auto test_string = "\xf0\x9f\x98\x80";
+
+    Vector<u32> processed_code_points;
+    decoder.process(test_string, [&](u32 code_point) {
+        processed_code_points.append(code_point);
+    });
+    EXPECT(processed_code_points.size() == 1);
+    EXPECT(processed_code_points[0] == 0x1F600);
+
+    EXPECT(decoder.to_utf8(test_string) == test_string);
+}