Reorganize internal solver tests

tests/solvers/__init__.py

@@ -0,0 +1,17 @@
+#  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 io import StringIO
+
+from miplearn.solvers import _RedirectOutput
+
+
+def test_redirect_output() -> None:
+    import sys
+
+    original_stdout = sys.stdout
+    io = StringIO()
+    with _RedirectOutput([io]):
+        print("Hello world")
+    assert sys.stdout == original_stdout
+    assert io.getvalue() == "Hello world\n"