t0021: skip EXPENSIVE test that is broken without SIZE_T_IS_64BIT

t0021: skip EXPENSIVE test that is broken without SIZE_T_IS_64BIT

One of the tests in t0021 writes a 2GB file and then roundtrips it
through the clean/sumdge filters. This test is broken on 32 bit
platforms because they typically don't handle files larger then
`SSIZE_MAX` well at all.

While our CI has a "linux32" job that should in theory hit this issue,
we never noticed it because we didn't use to run EXPENSIVE tests until
7a094d68a2 (ci: run expensive tests on push builds to integration
branches, 2026-05-08). And after that commit, the test does not fail but
instead hangs completely.

Ideally, we'd of course properly detect this situation and then test for
it. In practice, this turns out to be hard as the test failure are not
reliable as they often (but not always) run into ENOMEM errors.

Instead, skip the test altogether.

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