Don't ignore failed tests in Makefile

[tmeissner]

At the moment failed tests are ignored by the Makefile:

%: ../src/%.vhd ../src/pkg.vhd ../src/sequencer.vhd ../src/hex_sequencer.vhd %.sby
	mkdir -p work
	-sby --yosys "yosys -m ghdl" -f -d work/$@ $@.sby prove

This is be done because of a lot of PSL examples which should fail. Better would be a test process which is able to handle holding and failing assertions. Furthermore it would be good to verify the cycle the assertions fail.