x86/apic: Enable IRQs on vCPU0 for all tests

Enable IRQs on vCPU0 for all APIC sub-tests.  Many sub-tests want to
generate interrupts, and leaving IRQs disabled makes writing a new test
unnecessarily frustrating (guess who forgot to enable IRQs...).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221001011301.2077437-7-seanjc@google.com