gh-148868: Increase test coverage for `cmath.isinf` (#148869)

Lib/test/test_cmath.py

diff --git a/Lib/test/test_cmath.py b/Lib/test/test_cmath.py
index 389a3fa..a986fd6 100644
--- a/Lib/test/test_cmath.py
+++ b/Lib/test/test_cmath.py
@@ -516,6 +516,7 @@ def test_isinf(self):
         self.assertFalse(cmath.isinf(1j))
         self.assertFalse(cmath.isinf(NAN))
         self.assertTrue(cmath.isinf(INF))
+        self.assertTrue(cmath.isinf(-INF))
         self.assertTrue(cmath.isinf(complex(INF, 0)))
         self.assertTrue(cmath.isinf(complex(0, INF)))
         self.assertTrue(cmath.isinf(complex(INF, INF)))