Make internal_solvers into a fixture

tests/__init__.py

@@ -1,3 +1,18 @@
 #  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(),
+    ]