LibJS+LibUnicode: Port retrieving the system time zone to ICU

Tests/LibUnicode/CMakeLists.txt

@@ -4,6 +4,7 @@ set(TEST_SOURCES
     TestIDNA.cpp
     TestLocale.cpp
     TestSegmenter.cpp
+    TestTimeZone.cpp
     TestUnicodeCharacterTypes.cpp
     TestUnicodeNormalization.cpp
 )

Tests/LibUnicode/TestTimeZone.cpp

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2024, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+
+#include <AK/StringView.h>
+#include <LibCore/Environment.h>
+#include <LibUnicode/TimeZone.h>
+
+class TimeZoneGuard {
+public:
+    explicit TimeZoneGuard(StringView time_zone)
+        : m_time_zone(Core::Environment::get("TZ"sv))
+    {
+        MUST(Core::Environment::set("TZ"sv, time_zone, Core::Environment::Overwrite::Yes));
+    }
+
+    ~TimeZoneGuard()
+    {
+        if (m_time_zone.has_value())
+            MUST(Core::Environment::set("TZ"sv, *m_time_zone, Core::Environment::Overwrite::Yes));
+        else
+            MUST(Core::Environment::unset("TZ"sv));
+    }
+
+private:
+    Optional<StringView> m_time_zone;
+};
+
+TEST_CASE(current_time_zone)
+{
+    {
+        TimeZoneGuard guard { "America/New_York"sv };
+        EXPECT_EQ(Unicode::current_time_zone(), "America/New_York"sv);
+    }
+    {
+        TimeZoneGuard guard { "ladybird"sv };
+        EXPECT_EQ(Unicode::current_time_zone(), "UTC"sv);
+    }
+}