Update simgrid 3.19.1 -> 3.20 + parallel tests + enable darwin (#42721)

* simgrid: 3.19.1 -> 3.20

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/simgrid/versions.

These checks were done:

- built on NixOS
- /nix/store/cvyi6hvgc9rvgrnp7c028xrlzbl7jzb9-simgrid-3.20/bin/smpicc passed the binary check.
- /nix/store/cvyi6hvgc9rvgrnp7c028xrlzbl7jzb9-simgrid-3.20/bin/smpicxx passed the binary check.
- /nix/store/cvyi6hvgc9rvgrnp7c028xrlzbl7jzb9-simgrid-3.20/bin/smpirun passed the binary check.
- Warning: no invocation of /nix/store/cvyi6hvgc9rvgrnp7c028xrlzbl7jzb9-simgrid-3.20/bin/tesh had a zero exit code or showed the expected version
- /nix/store/cvyi6hvgc9rvgrnp7c028xrlzbl7jzb9-simgrid-3.20/bin/simgrid-colorizer passed the binary check.
- Warning: no invocation of /nix/store/cvyi6hvgc9rvgrnp7c028xrlzbl7jzb9-simgrid-3.20/bin/simgrid_update_xml had a zero exit code or showed the expected version
- /nix/store/cvyi6hvgc9rvgrnp7c028xrlzbl7jzb9-simgrid-3.20/bin/simgrid_convert_TI_traces passed the binary check.
- Warning: no invocation of /nix/store/cvyi6hvgc9rvgrnp7c028xrlzbl7jzb9-simgrid-3.20/bin/smpimain had a zero exit code or showed the expected version
- /nix/store/cvyi6hvgc9rvgrnp7c028xrlzbl7jzb9-simgrid-3.20/bin/graphicator passed the binary check.
- 6 of 9 passed binary check by having a zero exit code.
- 4 of 9 passed binary check by having the new version present in output.
- found 3.20 with grep in /nix/store/cvyi6hvgc9rvgrnp7c028xrlzbl7jzb9-simgrid-3.20
- directory tree listing: https://gist.github.com/edefc2f1a2e81412484edc2e45986e03
- du listing: https://gist.github.com/a44f7d57537e93152a8e6c569f8ed1ae

* simgrid: 3.19.1 -> 3.20 + add darwin

- Fix dependencies to enable build on darwin
- Add ctest flag to enable parallel testing

* Use simpler parallel testing logic

because it is already done in nixpkgs/pkgs/stdenv/generic/setup.sh