Support custom (pre-)commands for UnitTest and Coverage

[Paebbels]

In some cases, the test environment needs to be prepared by custom commands.

Allow a user to specify custom commands, that run before main steps in UnitTest or Coverage.

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Usecases:

• Setup dummy binaries like ghdl, so pyEDAA.CLITool can find a valid path.
  • create empty files (e.g. via touch)
  • copy dummy scripts to expected location (e.g. Python script emulating the ghdl CLI)
  • full ghdl installation

[umarcor]

Since UnitTesting and Coverage workflows have a single job each, it would be feasible to convert the steps into a composite action. Then, users who want to execute pre- or post- steps would write the matrix in their pipeline and call the composite action, instead of consuming the reusable workflow. That would solve #44 as well.

[umarcor]

Is there a demo of which steps would need to be added? I.e., some repo which uses a custom UnitTesting or Coverage workflow, instead of reusing the one from here as-is?