t0601: refactor tests to be shareable

In preparation for adding tests for the new `git refs optimize` command,
refactor the existing t0601 test suite to make its logic shareable.

Move the core test logic from `t0601-reffiles-pack-refs.sh` into a new
`pack-refs-tests.sh` file. Inside this new script, replace hardcoded
calls to "pack-refs" with the `$pack_refs` variable.

The original `t0601-reffiles-pack-refs.sh` script now becomes a simple
"driver". It is responsible for setting the default value of the
variable and then sourcing the test library.

This new structure follows the established pattern used for sharing
tests between `git-for-each-ref` and `git-refs list` and prepares the
test suite for the `refs optimize` tests to be added in a subsequent
commit.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: inosmeet