Actually fix the strptime tests

This has been a complicated issue to fix for a number of reasons.
The core of it is that the behavior is different between different
versions of macOS, some of which set possible-but-incorrect values.

This commit addresses the issue by always using our computation for
tm_wday and tm_yday on macOS. As a side-effect, strptime format
strings that specify %u and %j will no longer work on macOS.

docs/content/3.manual/manual.yml

@@ -1933,7 +1933,8 @@ sections:
           8601 datetime is `"%Y-%m-%dT%H:%M:%SZ"`.
 
           jq may not support some or all of this date functionality on
-          some systems.
+          some systems. In particular, the `%u` and `%j` specifiers for
+          `strptime(fmt)` are not supported on macOS.
 
         examples:
           - program: 'fromdate'