t3404: decouple some test cases from outcomes of previous test cases

t3404: decouple some test cases from outcomes of previous test cases

Originally, the `--preserve-merges` option of the `git rebase` command
piggy-backed on top of the `--interactive` feature. For that reason, the
early test cases were added to the very same test script that contains
the `git rebase -i` tests: `t3404-rebase-interactive.sh`.

However, since c42abfe7857 (rebase: introduce a dedicated backend for
--preserve-merges, 2018-05-28), the `--preserve-merges` feature got its
own backend, in preparation for converting the rest of the
`--interactive` code to built-in code, written in C rather than shell.

The reason why the `--preserve-merges` feature was not converted at the
same time is that we have something much better now: `--rebase-merges`.
That option intends to supersede `--preserve-merges`, and we will
probably deprecate the latter soon.

Once `--preserve-merges` has been deprecated for a good amount of time,
it will be time to remove it, and along with it, its tests.

In preparation for that, let's make the rest of the test cases in
`t3404-rebase-interactive.sh` independent of the test cases dedicated to
`--preserve-merges`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>