intTests: Add regression test for #2828.

Author: brianhuffman

intTests/test2828/README.txt

@@ -0,0 +1,15 @@
+File `adder_yosys.json` is produced using the directions in the
+"Analyzing Hardware Circuits using Yosys" section of the SAW User
+Manual.
+
+File `adder_tabby.json` is generated using the following commands in
+TabbyCAD:
+
+yosys> read -vhd adder.vhdl
+yosys> hierarchy -check -top add4
+yosys> write_json adder_tabby.json
+
+TabbyCAD/Verific insert some extra cells of type "$_BUF_" which are
+not present in `adder_yosys.json`. This proof script ensures that the
+TabbyCAD version can be read and proved equivalent to the plain Yosys
+version.