LibTest: Support death tests without child process cloning

A challenge for getting LibTest working on Windows has always
been CrashTest. It implements death tests similar to Google Test
where a child process is cloned to invoke the expression that
should abort/terminate the program. Then the exit code of the
child is used by the parent test process to verify if the
application correctly aborted/terminated due to invoking
the expression.

The problem was that finding an equivalent way to port Crash::run()
to Windows was not looking very likely as publicly exposed Win32/
Native APIs have no equivalent to fork(); however, Windows actually
does have native support for process cloning via undocumented NT
APIs that clever people reverse engineered and published, see
`NtCreateUserProcess()`.

All that being said, this `EXPECT_DEATH()` implementation avoids
needing to use a child process in general, allowing us to remove
CrashTest in favour of a single cross-platform solution for death
tests.

Libraries/LibTest/AssertionHandler.h

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2025, ayeteadoe <ayeteadoe@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Platform.h>
+#include <LibTest/Export.h>
+#include <setjmp.h>
+
+#ifndef AK_OS_WINDOWS
+#    define LIBTEST_SETJMP(env) sigsetjmp(env, 1)
+#    define LIBTEST_LONGJMP siglongjmp
+#else
+#    define LIBTEST_SETJMP(env) setjmp(env)
+#    define LIBTEST_LONGJMP longjmp
+#endif
+
+namespace Test {
+
+jmp_buf& assertion_jump_buffer();
+void set_assertion_jump_validity(bool);
+bool assertion_jump_validity();
+
+}