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

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

Test the transition points between all the currently listed Japanese
era name changes. This includes testing the transition between the
first year date and the second year date. This test will help test
the upcoming Japanese era name change.

Also fixes a fencepost error where the era name isn't properly parsed
by strptime in the last (partial) year of the era.

Example: if an era change happens in Feb 1990, and again in Aug 1995,
that's 5.5 years long, but the 0.5 year wasn't accounted for.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>