Unit test improvements

[jasoncouture]

During development of #302 tftp boots broke.

This caused the tests to hang, and never display the tftp failure panic.

I propose the following changes:

1. Add support for bootloader panic hander that uses the qemu exit port during tests.
2. Write a macro that splits the tests into _bios, _uefi, and _tftp

[phil-opp]

Sounds reasonable to me!