t: move tests exercising the "files" backend

t: move tests exercising the "files" backend

We still have a bunch of tests scattered across our test suites that
exercise on-disk files of the "files" backend directly:

  - t1301 exercises permissions of reflog files when the config
    "core.sharedRepository" is set.

  - t1400 exercises whether empty directories in the ref store are
    handled correctly.

  - t3200 exercises what happens when there are symlinks in the ref
    store.

  - t3400 also exercises what happens when ".git/logs" is a symlink.

All of these are inherently low-level tests specific to the "files"
backend. Move them into "t0600-reffiles-backend.sh" to reflect this.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>