AK+Tests: Destroy moved-from CallableWrappers

We won't attempt to destroy this wrapper later because m_kind gets set
to NullPointer. If any fields of the CallableType are not movable, this
results in a leak.

A test replicating the issue before the fix is added to a new
TestFunction.cpp file.

AK/Function.h

@@ -357,6 +357,7 @@ private:
         case FunctionKind::Inline:
         case FunctionKind::Block:
             other_wrapper->init_and_swap(m_storage, inline_capacity);
+            other_wrapper->~CallableWrapperBase();
             m_kind = other.m_kind;
             break;
         case FunctionKind::Outline:

Tests/AK/CMakeLists.txt

@@ -27,6 +27,7 @@ set(AK_TEST_SOURCES
     TestFind.cpp
     TestFixedArray.cpp
     TestFixedPoint.cpp
+    TestFunction.cpp
     TestFlyString.cpp
     TestFormat.cpp
     TestGenericLexer.cpp

Tests/AK/TestFunction.cpp

@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2026, Gregory Bertilson <gregory@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+
+#include <AK/Function.h>
+#include <AK/NonnullRefPtr.h>
+#include <AK/RefCounted.h>
+
+namespace {
+
+struct CopyOnly {
+    int& instance_count;
+
+    CopyOnly(int& count)
+        : instance_count(count)
+    {
+        instance_count++;
+    }
+
+    CopyOnly(CopyOnly const& other)
+        : instance_count(other.instance_count)
+    {
+        instance_count++;
+    }
+
+    ~CopyOnly()
+    {
+        instance_count--;
+    }
+};
+
+}
+
+TEST_CASE(move_construction_destroys_old_inline_wrapper)
+{
+    int instance_count = 0;
+
+    {
+        Function<void()> source = [captured = CopyOnly(instance_count)]() {
+            (void)captured;
+        };
+        EXPECT_EQ(instance_count, 1);
+
+        Function<void()> destination = move(source);
+        EXPECT_EQ(instance_count, 1);
+
+        source = nullptr;
+        EXPECT_EQ(instance_count, 1);
+    }
+
+    EXPECT_EQ(instance_count, 0);
+}
+
+TEST_CASE(move_assignment_destroys_old_inline_wrapper)
+{
+    int instance_count = 0;
+
+    {
+        Function<void()> source = [captured = CopyOnly(instance_count)]() {
+            (void)captured;
+        };
+        EXPECT_EQ(instance_count, 1);
+
+        Function<void()> destination;
+        destination = move(source);
+        EXPECT_EQ(instance_count, 1);
+
+        source = nullptr;
+        EXPECT_EQ(instance_count, 1);
+    }
+
+    EXPECT_EQ(instance_count, 0);
+}