t/pack-refs-tests: move the 'test_done' to callees

t/pack-refs-tests: move the 'test_done' to callees

In ac0bad0af4 (t0601: refactor tests to be shareable, 2025-09-19), we
refactored 't/t0601-reffiles-pack-refs.sh' to move all of the tests to
't/pack-refs-tests.sh', which became a common test suite which was also
used by 't/t1463-refs-optimize.sh'.

This also moved the 'test_done' directive to 't/pack-refs-tests.sh'.
Which inhibits additional tests from being added to either of the tests.
Let's move the directive out to both the tests, so that we can add
additional specific tests to them. Also the test flow logic shouldn't be
part of tests which can be embedded in other test scripts.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>