Permit to test when building bindings separately from main library (backport #1509)

[mergify]

🦟 Bug fix

Follow up of #1491 . #1491 enable the compilation of bindings separately from the main library, but does not permit to set BUILD_TESTING:BOOL=ON in those binding-only builds.

Summary

This PR permits to pass -DBUILD_TESTING:BOOL=ON in bindings-only builds, by:

• Ensuring that ${PROJECT_BINARY_DIR}/include/test_config.hh is generated also in bindings-only builds,
• Use CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR when creating the tests, as the CMAKE_CURRENT_SOURCE_DIR variable have a consistent value that is not influenced on which is the root directory of the CMake build.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

---

This is an automatic backport of pull request #1509 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 4ebe022 has failed:

On branch mergify/bp/sdf14/pr-1509
Your branch is up to date with 'origin/sdf14'.

You are currently cherry-picking commit 4ebe022c.
  (fix conflicts and run "git cherry-pick --continue")
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   python/CMakeLists.txt

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[scpeters]

I force-pushed an amended cherry-pick

[azeey]

@Mergifyio rebase

[mergify]

rebase

✅ Branch has been successfully rebased