SVA: add tests for #-# and #=#

This adds tests for the SVA followed-by operators.

Author: kroening

regression/verilog/SVA/followed-by2.desc

@@ -0,0 +1,10 @@
+CORE
+followed-by2.sv
+--bound 20
+^\[main\.p0\] always \(\(main\.a #-# main\.b\) iff \(not \(main\.a |-> \(not main\.b\)\)\)\): FAILURE: property not supported by BMC engine$
+^\[main\.p1\] always \(\(main\.a #=# main\.b\) iff \(not \(main\.a |=> \(not main\.b\)\)\)\): FAILURE: property not supported by BMC engine$
+^EXIT=10$
+^SIGNAL=0$
+--
+^warning: ignoring
+--

regression/verilog/SVA/followed-by2.sv

@@ -0,0 +1,7 @@
+module main(input clk, input a, input b);
+
+  // equivalences from 1800 2017 16.12.9
+  p0: assert property ((a #-# b) iff (not (a |-> not b)));
+  p1: assert property ((a #=# b) iff (not (a |=> not b)));
+
+endmodule