ColTypeDate: update test for "countdown" to match current code

Copy the unit test from "relative" for the positive case of "countdown",
since the code as implemented for "countdown" is identical to the code
for "relative".

I am including this commit, even though the next commit will override
it, to confirm my understanding of how the existing datetime styles
work. Initially I had gotten myself turned in circles because I misread
the "if now < date then add a - sign" logic as an absolute-value
calculation and badly misunderstood the behavior of "age" and "relative".

See #4013

Author: apoelstra

test/columns.test.py

@@ -417,7 +417,7 @@ def test_date_format_countdown(self):
         """Verify due.countdown formatting"""
         code, out, err = self.t("xxx rc.report.xxx.columns:id,due.countdown")
         self.assertRegex(out, r"1\s+")
-        self.assertRegex(out, r"2\s+\d+\S+")
+        self.assertRegex(out, r"2\s+[0-9.]+[hmin]+")
 
     def test_date_format_unrecognized(self):
         """Verify due.donkey formatting fails"""