test[dace]: Fix skip rules for dace optional dependency

[edopao]

This PR fixes the solution to skip testcases dependent on dace provided in #1635. The problem was that pytest.skip function was called within __init__ and/or conftest files in the test folder.

Additional changes:

• Moved all dace-related tests in dace_tests package folder.
• Introduced pytest marker requires_dace at package level, which needs to be included in all test files.
• Run ruff on test code in dace_tests package to cleanup and sort the import list.

[edopao]

@philip-paul-mueller Tested with Spack-build.

[philip-paul-mueller]

LGTM