gh-123811: Test that round(Decimal) can return signed zero (GH-124007)

(cherry picked from commit b46c65e)

Co-authored-by: Sergey B Kirpichev <[email protected]>

Author: skirpichev

Lib/test/test_decimal.py

@@ -2060,7 +2060,9 @@ def test_tonum_methods(self):
         #to quantize, which is already extensively tested
         test_triples = [
             ('123.456', -4, '0E+4'),
+            ('-123.456', -4, '-0E+4'),
             ('123.456', -3, '0E+3'),
+            ('-123.456', -3, '-0E+3'),
             ('123.456', -2, '1E+2'),
             ('123.456', -1, '1.2E+2'),
             ('123.456', 0, '123'),