t6300: refactor tests to be shareable

In preparation for adding tests for the new `git refs list` command,
refactor the existing t6300 test suite to make its logic shareable.

Move the core test logic from `t6300-for-each-ref.sh` into a new
`for-each-ref-tests.sh` file. Inside this new script, replace hardcoded
calls to "git for-each-ref" with the `$git_for_each_ref` variable.

The original `t6300-for-each-ref.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-blame` and `git-annotate` and prepares the test suite
for the `refs list` tests to be added in a subsequent commit.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: shejialuo <shejialuo@gmail.com>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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