t1301: use test_when_finished for cleanup

Refactor several test cases to use "test_when_finished" for cleanup. 1. For first of these, we used to clean-up outside the test, but instead let's use test_when_finished for that. 2. For the second, we used to leave "new" after we are done, but not use it at all later. Now we do clean up. 3. For the rest, these child.git test repositories used to follow "initialize what we are going to use to a known state before we use" pattern, which is not wrong per-se, but now we use "clean up the cruft we made after we are done" pattern, which may arguably be better simply because the test that makes cruft should know what cruft it created better than whatever comes later that may not know. Helped-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>