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

(cherry picked from commit 62792c8)

Co-authored-by: Manoj K M <manojkmdev24@gmail.com>

Author: manoj-km24

Lib/test/test_cmath.py

@@ -514,6 +514,7 @@ 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)))