Add executor generator for tests and benchmarks

[greole]

This PR adds a custom generator to generate available Executors. This allows to replace

    NeoFOAM::Executor exec = GENERATE(
        NeoFOAM::Executor(NeoFOAM::SerialExecutor {}),
        NeoFOAM::Executor(NeoFOAM::CPUExecutor {}),
        NeoFOAM::Executor(NeoFOAM::GPUExecutor {})
    );

by

NeoFOAM::Executor exec = GENERATE(allAvailableExecutor());

TODO:

• only yield every executor once. Currently, if compiled without GPUExecutor the fallback NeoFOAM::CPUExecutor is used.

[github-actions]

Deployed test documentation to https://exasim-project.com/NeoFOAM/Build_PR_251

[HenningScheufler]

LGTM