TST: Force Agg backend in test_openin_any_paranoid

On some CI, the Qt backend seems to be picked, but something is confused
and it fails due to an invalid `DISPLAY` variable. But this test doesn't
need an interactive backend, so don't use one.

Author: QuLogic

lib/matplotlib/tests/test_texmanager.py

@@ -71,5 +71,6 @@ def test_openin_any_paranoid():
          'plt.rcParams.update({"text.usetex": True});'
          'plt.title("paranoid");'
          'plt.show(block=False);'],
-        env={**os.environ, 'openin_any': 'p'}, check=True, capture_output=True)
+        env={**os.environ, 'MPLBACKEND': 'Agg', 'openin_any': 'p'},
+        check=True, capture_output=True)
     assert completed.stderr == ""