Fix strptime tests on macOS 10.12

Dates in 1900 are before the Unix epoch. We shouldn't make any promises
about how well they are supported, especially given that our time
support is a thin wrapper over the libc functions.

This changes the test to use dates after the epoch, which should fit
within both a signed and an unsigned 32-bit time_t.

Author: wtlangford

tests/optional.test

@@ -7,9 +7,9 @@
 
 # Check day-of-week and day of year computations
 # (should trip an assert if this fails)
-last(range(365 * 199)|("1900-03-01T01:02:03Z"|strptime("%Y-%m-%dT%H:%M:%SZ")|mktime) + (86400 * .)|strftime("%Y-%m-%dT%H:%M:%SZ")|strptime("%Y-%m-%dT%H:%M:%SZ"))
+last(range(365 * 199)|("1970-03-01T01:02:03Z"|strptime("%Y-%m-%dT%H:%M:%SZ")|mktime) + (86400 * .)|strftime("%Y-%m-%dT%H:%M:%SZ")|strptime("%Y-%m-%dT%H:%M:%SZ"))
 null
-[2099,0,10,1,2,3,6,9]
+[2169,0,10,1,2,3,1,9]
 
 # %e is not available on mingw/WIN32
 strftime("%A, %B %e, %Y")