strftime: exercise different times in Perl tests

This looks like it was the original intent.  --test.short will break
after just one time.
strftime: break build into linux/non-linux

And change the tests to follow.

In particular, strftime on OS X will yield slightly different results in
the face of invalid format specifiers.

Further, once you change a platform to use the Pure version, the
existing Perl tests will only complain about the fact that you are not
using the system version.