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>

Author: dscho

t/t5608-clone-2gb.sh

@@ -49,7 +49,7 @@ test_expect_success 'clone - with worktree, file:// protocol' '
 
 '
 
-test_expect_success SIZE_T_IS_64BIT 'set up repo with >4GB object' '
+test_expect_success SIZE_T_IS_64BIT,EXPENSIVE 'set up repo with >4GB object' '
 	large_blob_size=$((4*1024*1024*1024+1)) &&
 	git init --bare 4gb-repo &&
 	head_oid=$(test-tool synthesize pack \
@@ -60,7 +60,7 @@ test_expect_success SIZE_T_IS_64BIT 'set up repo with >4GB object' '
 	git -C 4gb-repo symbolic-ref HEAD refs/heads/main
 '
 
-test_expect_success SIZE_T_IS_64BIT 'clone >4GB object via unpack-objects' '
+test_expect_success SIZE_T_IS_64BIT,EXPENSIVE 'clone >4GB object via unpack-objects' '
 	# The synthesized pack has five objects, so a large unpack limit keeps
 	# fetch-pack on the unpack-objects path.
 	git -c fetch.unpackLimit=100 clone --bare \
@@ -76,7 +76,7 @@ test_expect_success SIZE_T_IS_64BIT 'clone >4GB object via unpack-objects' '
 	test "$source_blob" = "$clone_blob"
 '
 
-test_expect_success SIZE_T_IS_64BIT 'clone with >4GB object via index-pack' '
+test_expect_success SIZE_T_IS_64BIT,EXPENSIVE 'clone with >4GB object via index-pack' '
 	# Force fetch-pack to hand the pack to index-pack instead.
 	git -c fetch.unpackLimit=1 clone --bare \
 		"file://$(pwd)/4gb-repo" 4gb-clone-index &&