clone tests: rename t57* => t56*

When trying to find a good spot for testing clone with submodules, I
got confused where to add a new test file. There are both tests in t560*
as well as t57* both testing the clone command. t/README claims the
second digit is to indicate the command, which is inconsistent to the
current naming structure.

Rename all t57* tests to be in t56* to follow the pattern of the digits
as laid out in t/README.

It would have been less work to rename t56* => t57* because there are less
files, but the tests in t56* look more basic and I assumed the higher the
last digits the more complicated niche details are tested, so with the patch
now it looks more in order to me.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>