genirq/test: Ensure CPU 1 is online for hotplug test

genirq/test: Ensure CPU 1 is online for hotplug test

It's possible to run these tests on platforms that think they have a
hotpluggable CPU1, but for whatever reason, CPU1 is not online and can't be
brought online:

    # irq_cpuhotplug_test: EXPECTATION FAILED at kernel/irq/irq_test.c:210
    Expected remove_cpu(1) == 0, but
        remove_cpu(1) == 1 (0x1)
CPU1: failed to boot: -38
    # irq_cpuhotplug_test: EXPECTATION FAILED at kernel/irq/irq_test.c:214
    Expected add_cpu(1) == 0, but
        add_cpu(1) == -38 (0xffffffffffffffda)

Check that CPU1 is actually online before trying to run the test.

Fixes: 66067c3c8a1e ("genirq: Add kunit tests for depth counts")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/all/20250822190140.2154646-7-briannorris@chromium.org