x86/apic: Restore APIC to original state after every sub-test

Restore the APIC to its original state after every APIC sub-test.  Many
of the tests already do (parts of) this manually, while others do not.
Always restore to dedup code and to avoid cross-test dependencies.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221001011301.2077437-6-seanjc@google.com