Fix imports of fixture-decorated function_parameters function in tests with Ruff

[mcw92]

The fixture-decorated function function_parameters is used in multiple tests for requesting different combinations of benchmark function name and the corresponding global optimum:

@pytest.fixture(
    params=[
        ("rosenbrock", 0.0),
        ("step", -25.0),
        ("quartic", 0.0),
        ("rastrigin", 0.0),
        ("griewank", 0.0),
        ("schwefel", 0.0),
        ("bisphere", 0.0),
        ("birastrigin", 0.0),
        ("bukin", 0.0),
        ("eggcrate", -1.0),
        ("himmelblau", 0.0),
        ("keane", 0.6736675),
        ("leon", 0.0),
        ("sphere", 0.0),  # (fname, expected)
    ]
)
def function_parameters(request):
    """Define benchmark function parameter sets as used in tests."""
    return request.param

When moving the definition of this function to a separate utility module file and importing from this file into the actual tests, ruff falsely recognizes this as an unused import and thus will remove the line:

ruff check test_cmaes.py 
test_cmaes.py:8:25: F401 [*] `tests.utils.function_parameters` imported but unused
test_cmaes.py:11:16: F811 Redefinition of unused `function_parameters` from line 8

This should not be the case and fixed to avoid duplicate code by repeating the definition of function_parameters in each separate test file to be executed.