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.