Setup simulation-based testing flow

[Scheremo]

This PR fills in the gaps for running chimera-sdk tests with simulation tools (VCS, Verilator).

Added

• Variables TEST_MODE, SIMULATON_BINARY, PRELOAD_MODE which control the simulation launch

Changed

• add_chimera_test now invokes SIMULATION_BINARY with plusargs for BINARY and PRELMODE corresponding to the chimera / cheshire testbench convention.

[Xeratec]

Thanks for the contribution! In principle looks good, but I have a few comments. Most important, I would like to support other simulation platforms such as GVSoC in the future.

Furthermore, if you have some time and extend the Sphinx documentation with a minimal explanation of how to run the test, this would be great, but is not strictly necessary.

[Xeratec]

Thanks for the feedback. I was thinking on another level of abstraction, and most comments are resolved now. Open points:

• Rename "Simulation Binary"
• List of available models

[Scheremo]

Fixed the open points, and actually flip-flopped on test building; I think it's not a bad idea to make sure the tests build in any case (Ensures the toolchain works, the code is not deeply broken,...), so I reenabled the default building of tests, but added a switch for adding them to the ctest pipeline, which is now contingent on TEST_MODE being "simulation" - this can be easily extended for gvsoc or other platforms.

[Xeratec]

All good, except the formatting of the RST documentation for the CMake add_chimera_subdirectories function.

[Xeratec]

LGTM