Added note to "make test": use git checkout of iverilog

tests/realmath/run-test.sh

@@ -15,6 +15,7 @@ for ((i = 0; i < 100; i++)); do
 	iverilog -o uut_${idx}_tb uut_${idx}_tb.v uut_${idx}.v uut_${idx}_syn.v
 	./uut_${idx}_tb | tee uut_${idx}.err
 	if test -s uut_${idx}.err; then
+		echo "Note: Make sure that 'iverilog' is an up-to-date git checkout of icarus verilog."
 		exit 1
 	fi
 	rm -f uut_${idx}.err