C API: Split test suite, headers into C++-independent component

[roberth]

Is your feature request related to a problem? Please describe.

Many C API tests don't use the internal details. Nonetheless, they are part of the same test suite. As a consequence:

• Many C API tests are unnecessarily recompiled when developing Nix itself, even without touching the api files.
• The C API test suite can not be reused by alternate Nix implementations.

Describe the solution you'd like

1. Move internals-dependent tests into separate files, or remove/replace unnecessary calls/fields accesses

2. Move the C API tests into a separately compiled subproject

3. Move the C API headers and tests into a separate repository

Describe alternatives you've considered

• Stop before (2) or (3) or save those for after the stability milestone.

Additional context

Priorities

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[Ericson2314]

I think with the Meson work, doing 1 and 2 should be very straightforward and easy.