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

[DJ Delorie]

TAMUKI Shoichi <tamuki@linet.gr.jp> writes:
> This test case is intended to check whether strptime can correctly
> parse %EY, so it would be nice to rename it to tst-strptime4.c.

The test case was originally written to further test the strftime
handling of the upcoming era change, but I don't care what the name of
the test is ;-)

I added the strptime check "just because" and only found the strptime
bug after that.

> The test case to check whether strftime can correctly format %EY
> already exists as tst-strftime2.c.

Yup, this is an additional test.

I see no harm in having overlapping test coverage, especially if the
tests approach the problem in different ways.  New coverage can be added
to whichever test the submitter finds more comfortable editing, too.
Personally, I'm not a fan of logic-driven test data.  I prefer
table-driven since I don't need to worry as much about bugs in the test
itself, and it's easier to list unusual test cases than try to encode
them in logic.