rebase --apply: restore some tests

rebase --apply: restore some tests

980b482d28 ("rebase tests: mark tests specific to the am-backend with
--am", 2020-02-15) sought to prepare tests testing the "apply" backend
in preparation for 2ac0d6273f ("rebase: change the default backend
from "am" to "merge"", 2020-02-15). However some tests seem to have
been missed leading to us testing the "merge" backend twice. This
patch fixes some cases that I noticed while adding tests to these
files, I have not audited all the other rebase test files. I've
reworded a couple of the test descriptions to make it clear which
backend they are testing.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>