add basic tests for os::Event/SystemEvent functionality

ams: support building unit test programs on windows/linux/macos

test/svc: add test for thread create

test/svc: add test for thread pinning/SynchronizePreemptionState

unit testing: catch -> doctest (faster compile, thread-safe)

kern/test: add some scheduler tests (yields work correctly, all non-special priorities are cooperative/not pre-emptive

hos: allow turning off ams extension hard-reqs for unit testing

test: add tests for SetMemoryPermission

kern/test: add wip qemu-virt board support to mesosphere