Define tests for the interfaces

[Paebbels]

As requested by @LarsAsplund, we should also provide tests with these interfaces.

So let us discuss what we need and how we do it. Here is an initial proposal for discussion from my side.

Goals

• Check if packages analyze.
  • Check for syntax errors.
  • Check for type and constraint errors.
• Check if packages elaborate.
  • Use a package and it's types and views.
  • Use interfaces in port maps.

Out of scope

• Check if bits driven on one side travel through the interface properly from A to B.

Testbench Minimum Requirements

• Test harness made of top-level entity + architecture
• Two component for each side of the interface

Testbench Execution Environment

• Testbench description files
• Should run in CI, when a tool supports this on GitHub Actions or Travis.

Optional

• Tests based on VUnit

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/cc @LarsAsplund, @eine, @JimLewis