Beautify test formatting

Headers can be added by passing 0 as test. This will add a newline before the line too, so it looks like this:

    Testing OUTI ..............................................PASS
    Testing OUTD ..............................................PASS
    Testing OTIR ..............................................PASS
    Testing OTDR ..............................................PASS
    
    --- Interrupt tests
    Testing mode 1 interrupts .................................PASS
    Testing mode 2 interrupts .................................PASS