Add unit test infrastructure

[tstirrat]

Based on https://github.com/untoxa/gbdk_unit_test, but uses pytest as the test runner:

• More expression in assertions
• Much easier to debug

General struture:

• Makefile set up so that for any rom.c, you can make build/rom.gb
• snapshot fixture, that will build + run + snapshot before each test (use indirect=True with parametrize())

Example:

# my_rom.c contains a simple rom that calls EMU_BREAKPOINT

# snapshot calls "make build/my_rom.gb" and then runs the rom and captures the snapshot
@pytest.mark.parametrize('snapshot', ['build/my_rom.gb'], indirect=True)
def it_does_something(snapshot):
    rAUD1LOW = get(snapshot, 'IO_REG', '_NR13_REG')
    assert rAUD1LOW == 0x72