t5608: mark >4GB tests as EXPENSIVE

t5608: mark >4GB tests as EXPENSIVE

Even with precomputed pack constants that reduced the helper's
runtime from minutes to seconds, the >4GB clone tests still take
200-850 seconds across CI jobs. The bottleneck is no longer the
pack generation but the clone operations themselves: transporting,
unpacking, and indexing 4 GiB of data through unpack-objects and
index-pack is inherently expensive.

As Jeff King pointed out [1], t5608 alone takes 160 seconds on his
laptop while the rest of the entire test suite finishes in under 90
seconds, and the test's disk footprint (4+ GiB source repo, then
two clones) is problematic for developers who use RAM disks for
their trash directories.

Gate the >4GB tests on the EXPENSIVE prereq (which requires
GIT_TEST_LONG to be set) in addition to SIZE_T_IS_64BIT, keeping
them out of normal local test runs.

[1] https://lore.kernel.org/git/20260501063805.GA2038915@coredump.intra.peff.net/

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>