Make tests/everest/test_api use a shared multiobj storage instead of saved seba db

[yngve-sk]

Make it do exactly the same, but using the seba db from the shared case. Should be set up similar to the way we have a shared case for snake oil in some ERT tests.

Definition of done: Remove all use of mock_SebaSnapshot in test_api.py, it should instead get the seba snapshot from the seba db in the shared case.

Possible starting point:

@pytest.fixture
def _shared_multiobj_case(request, monkeypatch):
    """This fixture will run the multiobj case to populate storage,
    this is quite slow, but the results will be cached. If something comes
    out of sync, clear the cache and start again.
    """

    test_data_path = Path(
        relpath("..", "..", "test-data", "everest", "math_func")
    ).resolve()

    output_path = request.config.cache.mkdir(
        "multiobj_data" + os.environ.get("PYTEST_XDIST_WORKER", "")
    )

    if not os.path.exists(output_path / "test_data"):
        os.mkdir("test_data")
        shutil.copy(test_data_path / "config_multiobj.yml", "./test_data/config_multiobj.yml")
        shutil.copytree(test_data_path / "jobs", "./test_data/jobs")
        sys.argv = ["run", "--new-run", "./test_data/config_multiobj.yml"]
        from everest.bin.everest_script import everest_entry

        everest_entry()

    monkeypatch.chdir(output_path / "test_data")

    yield output_path / "test_data"


@pytest.fixture(
    name="copy_multiobj_case",
    params=[
        pytest.param(0, marks=pytest.mark.xdist_group(name="snake_oil_case_storage"))
    ],
)
def fixture_multiobj_case(_shared_multiobj_case, tmp_path, monkeypatch):
    monkeypatch.chdir(tmp_path)
    shutil.copytree(_shared_multiobj_case, "test_data")
    monkeypatch.chdir("test_data")