t: make many tests depend less on the refs being files

t: make many tests depend less on the refs being files

Many tests are very focused on the file system representation of the
loose and packed refs code. As there are plans to implement other
ref storage systems, let's migrate these tests to a form that test
the intent of the refs storage system instead of it internals.

This will make clear to readers that these tests do not depend on
which ref backend is used.

The internals of the loose refs backend are still tested in
t1400-update-ref.sh, whereas the tests changed in this patch focus
on testing other aspects.

This patch just takes care of many low hanging fruits. It does not
try to completely solves the issue.

Helped-by: Stefan Beller <sbeller@google.com>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>