Tweak the overlay related `insert-before-markers' tests

Tweak the overlay related `insert-before-markers' tests

* test/src/buffer-tests.el (test-overlay-insert-before-markers-empty):
Move code down to the other tests related to insertion.  Test all
front/rear insert combinations.  To make the test more clear, at least
to me, hard code all character positions.
(test-overlay-insert-before-markers-at-start): For both front-advance
modes verify that `insert-before-markers' at and overlay's start
advances it.
(test-overlay-insert-before-markers-at-end): For both rear-advance
modes test that `insert-before-markers' at an overlay's end advances
it.
(test-overlay-insert-before-markers-non-empty): Delete, replaced by
the two tests above.