arch: tests: Document interrupt delivery behavior after arch_irq_disable()

Upcoming changes from Andrew that add a global timeout to the kernel
broke because of some voodoo behavior in the kernel/context test.  It
will use arch_irq_disable() on the timer interrupt directly to prevent
interrupts and measure timekeeping in their absence.  But some
architectures[1] don't reliably deliver interrupts that arrive, which
means that a running timeout that exists across this period will
result in a corrupt timeout queue.

Document that rule for architectures, move the offending test to the
end of the test suite (to minimize the chance of interacting with
other test code) and put a giant warning about the situation on it.
Long term, we may want to rework this test to do its job in other
ways.

[1] On x86, the interrupt disable happens at the IO-APIC level, while
interrupt latching and delivery is downstream in each CPU's Local
APIC.  An IO-APIC masked interrupt is completely invisible to the APIC
and can never be delivered once the line goes low.

Fixes #31333

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>