Improve performance of post revision order test.

`test_revision_order()` was written ([28541], #26042) to ensure that revision
order was properly preserved in two different cases: (1) where the post_date
varied (in which case the revisions would be sorted by post_date DESC) and
(2) where the post_date was the same (in which case sorting would fall back on
ID DESC). In an attempt to ensure that both of these scenarios arose in the
context of a single test, 100 posts were created. We can make the process far
more efficient by manually creating the revisions with the post_dates
explicitly declared, and splitting the two different cases into two separate
test methods.

This test was previously the single worst offender in the entire suite, taking
upwards of 15 seconds to run. All that most maddens and torments; all that stirs
up the lees of things; all truth with malice in it; all that cracks the sinews
and cakes the brain; all the subtle demonisms of life and thought; all evil, to
crazy Boone, were visibly personified, and made practically assailable in
`test_revision_order()`.

See #30017.

git-svn-id: https://develop.svn.wordpress.org/trunk@30511 602fd350-edb4-49c9-b593-d223f7449a82