[v4] Fix strptime era handling, add more era tests [BZ #24394]

[Florian Weimer]

* Carlos O'Donell:

> .. and tst-strftime2.c is hideous, a terrible test with no comments
> whose logic cannot be followed.

I think it's not too bad.  It's one of these combination tests.  I
can't read Japanese, so I have trouble understanding what it exactly
tests, but any test trying to get coverage like this will looke pretty
similar.