der: add DateTime leap year datetime.proptest-regressions (#1829)

Author: dishmaker

der/tests/datetime.proptest-regressions

@@ -6,3 +6,5 @@
 # everyone who runs the test benefits from these saved cases.
 cc 00dbea7e90761c16aa20e2fbf7ffad420da0c84d4ed4e6df123de03c9b4567e5 # shrinks to year = 1970, month = 1, day = 1, hour = 0, min = 60, sec = 0
 cc 3b0bd01ef4cad6bea0a287f9cdcd56bad186125ec388d204f6afcd193ca12c39 # shrinks to year = 1970, month = 1, day = 1, hour = 0, min = 0, sec = 60
+cc b15c8e6f894bfcb36a5d6b6e03f52b2b565dde7feafd44bf53941f9e9b1e67f4 # shrinks to year = 6580, month = 2, day = 29, hour = 0, min = 0, sec = 0
+cc ef4a0bb7c5ed9d58d2df8a731d3b9c1bc736ef0bd1f08a8c5f23ae804c0b6c24 # shrinks to year = 9600, month = 2, day = 29, hour = 0, min = 0, sec = 0