tests: Add trivial liberty -unit_delay test

tests/liberty/run-test.sh

@@ -8,3 +8,8 @@ for x in *.lib; do
 	../../yosys-filterlib -verilogsim $x > $x.verilogsim
 	diff $x.filtered $x.filtered.ok && diff $x.verilogsim $x.verilogsim.ok
 done
+
+for x in *.ys; do
+  echo "Running $x.."
+  ../../yosys -q -s $x -l ${x%.ys}.log
+done

tests/liberty/unit_delay.ys

@@ -0,0 +1,3 @@
+# Nothing gets imported: the file lacks timing data
+read_liberty -wb -unit_delay normal.lib
+select -assert-none =*/t:$specify*