arm: add host uart driver tests

All host tests currently pass
Some async refactors may not work well on device, will try later

arm/fake_uart.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <cstdint>
+#include <span>
+
+#include "buffer.h"
+
+buffer FakeUart_Drain(uint16_t size);
+void FakeUart_Feed(std::span<const std::byte> data);
+void FakeUart_Reset();