t5701: add setup test to remove side-effect dependency

t5701: add setup test to remove side-effect dependency

Currently, the "test capability advertisement" test creates some files
with expected content which are used by other tests below it.

To remove that side-effect from this test, let's split up part of
it into a "setup"-type test which creates the files with expected content
which gets reused by multiple tests. This will be useful in a following
commit.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>