Tests: Add coverage for sys$alarm() canceling a stale timer

This is a regression test to validate the functionality that was
reported broken in #9071, where the kernel would spin attempting
to cancel a stale timer.

Tests/Kernel/TestKernelAlarm.cpp

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2021, Brian Gianforcaro <bgianf@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Time.h>
+#include <LibTest/TestCase.h>
+#include <unistd.h>
+
+// Regression test for issues #9071
+// See: https://github.com/SerenityOS/serenity/issues/9071
+TEST_CASE(regression_inifinite_loop)
+{
+    constexpr auto hour_long_timer_value = Time::from_seconds(60 * 60);
+
+    // Create an alarm timer significantly far into the future.
+    auto previous_time = alarm(hour_long_timer_value.to_seconds());
+    EXPECT_EQ(previous_time, 0u);
+
+    // Update the alarm with a zero value before the previous timer expires.
+    previous_time = alarm(0);
+    EXPECT_EQ(previous_time, hour_long_timer_value.to_seconds());
+
+    // Update the alarm with a zero value again, this shouldn't get stuck
+    // in an infinite loop trying to cancel the previous timer in the kernel.
+    previous_time = alarm(0);
+    EXPECT_EQ(previous_time, 0u);
+}