icebox_vlog: improve module declaration and unit testing

[X-Illuminati]

Resolves #328

• icebox_vlog: use proper Verilog-2001 module declarations
• icebox_vlog: add basic test cases

First commit changes iceblox_vlog to output Verilog-2001 module declarations by default. An additional command line option (-C) supports output of Verilog-1995 module declarations for backwards compatibility.

Second commit adds test cases that try out different combinations of commandline options for iceblox_vlog and ensure they parse correctly with iverilog and yosys. I didn't see any other obvious regression tests, so I added these to support development in the future.

[X-Illuminati]

It seems clear to me that icebox_vlog is largely superceded by the yosys "write_verilog" command. This change may be seen mostly as a personal journey in understanding, but it may prove useful for continuing to work with unmaintained scripts in other projects.

[X-Illuminati]

@gatecat I hope you can review this or assign to a relevant reviewer.