t6301: write invalid object ID via `test-tool ref-store`

One of the tests in t6301 verifies that the reference backend correctly
warns about the case where a reference points to a non-existent object.
This is done by writing the object ID into the loose reference directly,
which is quite intimate with how the files backend works.

Refactor the code to instead use `test-tool ref-store` to write the
reference, which is backend-agnostic.

There are two more tests in this file which write loose files directly,
as well. But both of them are indeed quite specific to the loose files
backend and cannot be easily ported to other backends. We thus mark them
as requiring the REFFILES prerequisite.

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

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