Reorganize internal solver tests

tests/__init__.py

@@ -1,18 +0,0 @@
-#  MIPLearn: Extensible Framework for Learning-Enhanced Mixed-Integer Optimization
-#  Copyright (C) 2020-2021, UChicago Argonne, LLC. All rights reserved.
-#  Released under the modified BSD license. See COPYING.md for more details.
-from typing import List
-
-import pytest
-
-from miplearn import InternalSolver, GurobiPyomoSolver, GurobiSolver
-from miplearn.solvers.pyomo.xpress import XpressPyomoSolver
-
-
-@pytest.fixture
-def internal_solvers() -> List[InternalSolver]:
-    return [
-        GurobiPyomoSolver(),
-        GurobiSolver(),
-        XpressPyomoSolver(),
-    ]