NeverDestroyed: Add tests

Problem:
- It is difficult to refactor because there are no tests to bind the
functionality.
- Arguments are not forwarded correctly to the constructor.

Solution:
- Add tests.
- Change constructor to take forwarding references.

AK/NeverDestroyed.h

@@ -38,7 +38,7 @@ class NeverDestroyed {
 
 public:
     template<typename... Args>
-    NeverDestroyed(Args... args)
+    NeverDestroyed(Args&&... args)
     {
         new (storage) T(forward<Args>(args)...);
     }

AK/Tests/TestNeverDestroyed.cpp

@@ -0,0 +1,95 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/TestSuite.h>
+
+#include <AK/NeverDestroyed.h>
+#include <AK/StdLibExtras.h>
+
+struct Counter {
+    Counter() = default;
+
+    ~Counter() { ++num_destroys; }
+
+    Counter(const Counter&)
+    {
+        ++num_copies;
+    }
+
+    Counter(Counter&&) { ++num_moves; }
+
+    int num_copies {};
+    int num_moves {};
+    int num_destroys {};
+};
+
+TEST_CASE(should_construct_by_copy)
+{
+    Counter c {};
+    AK::NeverDestroyed<Counter> n { c };
+
+    EXPECT_EQ(1, n->num_copies);
+    EXPECT_EQ(0, n->num_moves);
+}
+
+TEST_CASE(should_construct_by_move)
+{
+    Counter c {};
+    AK::NeverDestroyed<Counter> n { AK::move(c) };
+
+    EXPECT_EQ(0, n->num_copies);
+    EXPECT_EQ(1, n->num_moves);
+}
+
+TEST_CASE(should_not_destroy)
+{
+    Counter* c = nullptr;
+    {
+        AK::NeverDestroyed<Counter> n {};
+        c = &n.get();
+    }
+    EXPECT_EQ(0, c->num_destroys);
+}
+
+TEST_CASE(should_provide_dereference_operator)
+{
+    AK::NeverDestroyed<Counter> n {};
+    EXPECT_EQ(0, n->num_destroys);
+}
+
+TEST_CASE(should_provide_indirection_operator)
+{
+    AK::NeverDestroyed<Counter> n {};
+    EXPECT_EQ(0, (*n).num_destroys);
+}
+
+TEST_CASE(should_provide_basic_getter)
+{
+    AK::NeverDestroyed<Counter> n {};
+    EXPECT_EQ(0, n.get().num_destroys);
+}
+
+TEST_MAIN(NeverDestroyed)