test(unistd): Add alarm(2) test

Introduce a new test for the alarm(2) system call.
This test verifies the behavior of setting, cancelling,
and re-arming alarms, ensuring correct signal delivery
and return values.

tests/expected/bins_dynamic/unistd/alarm.stdout

@@ -0,0 +1,4 @@
+alarm(0) baseline: ok
+alarm(1) fires: ok
+alarm(0) cancel: ok
+alarm re-arm returns remaining: ok

tests/expected/bins_static/unistd/alarm.stdout

@@ -0,0 +1,4 @@
+alarm(0) baseline: ok
+alarm(1) fires: ok
+alarm(0) cancel: ok
+alarm re-arm returns remaining: ok

tests/unistd/alarm.c

@@ -0,0 +1,80 @@
+/*
+ * Tests for alarm(2) - POSIX: https://pubs.opengroup.org/onlinepubs/9799919799/functions/alarm.html
+ *
+ * Verifies:
+ *  1. alarm(0) with no pending alarm returns 0
+ *  2. alarm(1) delivers SIGALRM after ~1 second (tested via pause())
+ *  3. alarm(0) cancels a pending alarm
+ *  4. re-arming alarm returns the remaining seconds from the previous alarm
+ */
+
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include "test_helpers.h"
+
+static volatile sig_atomic_t alarm_count = 0;
+
+static void handler(int sig) {
+    (void)sig;
+    alarm_count++;
+}
+
+int main(void) {
+    struct sigaction sa;
+    sa.sa_handler = handler;
+    sa.sa_flags = 0;
+    sigemptyset(&sa.sa_mask);
+    int r = sigaction(SIGALRM, &sa, NULL);
+    ERROR_IF(sigaction, r, == -1);
+
+    /* alarm(0) with no existing alarm must return 0 */
+    unsigned prev = alarm(0);
+    UNEXP_IF(alarm, (int)prev, != 0);
+    puts("alarm(0) baseline: ok");
+
+    /* alarm(1): SIGALRM must fire; pause() blocks until the signal arrives */
+    alarm_count = 0;
+    alarm(1);
+    pause();
+    if (alarm_count != 1) {
+        fprintf(stderr, "SIGALRM did not fire (count=%d)\n", (int)alarm_count);
+        return EXIT_FAILURE;
+    }
+    puts("alarm(1) fires: ok");
+
+    /* alarm(0) must cancel a pending alarm and return remaining secs > 0 */
+    alarm_count = 0;
+    alarm(10);
+    unsigned remaining = alarm(0);
+    if (remaining == 0) {
+        fprintf(stderr, "alarm(0) cancel: expected remaining > 0\n");
+        return EXIT_FAILURE;
+    }
+    /* Wait longer than original alarm; SIGALRM must NOT fire */
+    sleep(2);
+    if (alarm_count != 0) {
+        fprintf(stderr, "SIGALRM fired after alarm(0) cancel\n");
+        return EXIT_FAILURE;
+    }
+    puts("alarm(0) cancel: ok");
+
+    /*
+     * Re-arming: arm with 10s, sleep 1s, then re-arm with 3s.
+     * The return value must be the remaining time (~9s) which is > 0.
+     */
+    alarm_count = 0;
+    alarm(10);
+    sleep(1);
+    remaining = alarm(3);
+    if (remaining == 0) {
+        fprintf(stderr, "re-arm: expected remaining > 0, got 0\n");
+        return EXIT_FAILURE;
+    }
+    alarm(0); /* disarm so SIGALRM doesn't fire after the test */
+    puts("alarm re-arm returns remaining: ok");
+
+    return EXIT_SUCCESS;
+}