Align meson library naming and make tests optional

[robin-dahl]

Summary

This PR improves ddX's meson integration for downstream projects such as tblite and xTB.

• Aligns the meson library name with CMake by producing libddx instead of libddX, addressing ddX library name differs between meson and CMake build #179.
• Adds a tests option, enabled by default, allowing downstream projects to exclude ddX's internal test targets with -Dddx:tests=false.

The optional test suite is needed because ddX's tests are otherwise added unconditionally to the parent project's default build graph. In particular, the symlink-based ddx_driver_testing source currently fails to compile on Windows when ddX is consumed through tblite.

The default standalone behavior remains unchanged, and the complete test suite passes with tests enabled.

[mnottoli]

I was not entirely satisfied by skipping the failing test on Windows rather than fixing it, so I removed the symlink.
Since we already have a python wrapper run_test.py, we can just point it to the right executable (ddx_driver) rather than making the symlink.

Since we don't have the symlink anymore, the meson_test_wrapper.in and the workaround to create bash scripts are no longer needed.

Can you test if everything works also when ddX is a subproject?

If you are satisfied with the modifications, I will merge the PR and tag the new ddX version.

[robin-dahl]

@mnottoli I just tested it, and it seems like everything is working. Thank you!