Add PSL tests

[Paebbels]

@tmeissner has a repository for testing GHDL's PSL capabilities.
Maybe this should be integrated too.

https://github.com/tmeissner/psl_with_ghdl/

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/cc @umarcor

[umarcor]

I believe this is well covered already.

Torsten is the contributor of reference with regard to PSL (https://ghdl.github.io/ghdl/licenses.html#list-of-contributors), so we do have fluent communication when required.
I helped him setup CI in his repos (https://github.com/tmeissner/psl_with_ghdl/blob/master/.github/workflows/formal.yml#L13). We either chat, he opens some issue, a PR, we ask Tristan... It depends on the specific problem. It is desirable to keep that CI in Torsten's repos because PSL and formal verification cannot be tested with PSL only. Yosys and SymbiYosys are required.

Initially Yosys and SymbiYosys were built and containers were maintained in ghdl/docker. However, that content didn't really belong in there, so it was moved to hdl/containers. That is what users of PSL and formal verification with open source tools should consume. It's the fastest path for testing bleeding-edge GHDL along with Yosys and SymbiYosys.

Currently:

• There are PSL tests in GHDL's testsuite (https://github.com/ghdl/ghdl/tree/master/testsuite/synth), which do not need Yosys or SymbiYosys.
• There are some tests in ghdl-yosys-plugin (https://github.com/ghdl/ghdl-yosys-plugin/tree/master/testsuite/formal). Yosys, SymbiYosys and formal tools are retrieved from hdl/containers, and GHDL is picked from ghdl/docker, but the plugin itself is built from sources.
• Torsten has this other repo which uses containers from hdl/containers: https://github.com/tmeissner/formal_hw_verification.

Note that there is a relatively complex cross-relation between ghdl/ghdl, ghdl/docker, ghdl/ghdl-yosys-plugin, hdl/containers, setup-ghdl-ci, Torsten's repos, etc. A similar complexity exists with regard to co-simulation (ghdl-cosim, VUnit, cocotb, hdl/containers, setup-ghdl-ci...). Other subsets are/will be Synthesis/PnR (fomu-workshop, NEORV32, microwatt, etc.) and ASIC tooling. See section "Images including multiple tools" in https://hdl.github.io/containers/#_tools_and_images, or the graphs in https://hdl.github.io/containers/dev/index.html#_graphs for understanding which are the 3-5 domains.
Some day I will hopefully have time for actually doing a talk/presentation about that... Meanwhile, we can have a video meeting if you wish.