t7508: skip EXPENSIVE test that is broken without SIZE_T_IS_64BIT

t7508: skip EXPENSIVE test that is broken without SIZE_T_IS_64BIT

One of the tests in t7508 is marked as EXPENSIVE because it ends up
creating and adding files that are multiple gigabytes in size. This
takes a while to complete, hence the EXPENSIVE prerequisite.

Besides being expensive though the test can only work on systems where
`size_t` is at least 64 bit. This is because one of the created files
is larger than 4GB, and because Git tracks object size via `size_t` it
will eventually blow up.

This test has also been blowing up in the "linux32" CI job in GitHub
Workflows since 7a094d68a2 (ci: run expensive tests on push builds to
integration branches, 2026-05-08). But that job doesn't only fail, it
also hangs, and that has been concealing the failure.

Fix the issue by marking the test as requiring 64 bit `size_t`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>