improve VUnit generation to support varying generic parameters while still running the same testbench.

[nathanaelhuffman]

It would be nice if we could run the same TB against THs configured differently (different genercis). Either teach VUnit how to do that, do some sort of pre-processing and then point VUnit to a set of generated TBs which were identical aside from the TH.

This is possible in vunit, but we need some thinking for how this would be exposed up to the build system so that things generate and are reported correctly.