Add test to verify that Param<> copies refer to the same Parameter
Add test to verify that Param<> copies refer to the same Parameter

This was always implicitly supported, but never tested. (I’m assuming
this is desired behavior.)

psuriana/Halide,psuriana/Halide,psuriana/Halide,psuriana/Halide,kgnk/Halide,kgnk/Halide,psuriana/Halide,kgnk/Halide,psuriana/Halide,kgnk/Halide,psuriana/Halide,kgnk/Halide,kgnk/Halide,kgnk/Halide,kgnk/Halide