read-tree tests: check "diff-files" exit code on failure

read-tree tests: check "diff-files" exit code on failure

Fix an issue with the exit code of "diff-files" being ignored, which
has been ignored ever since these tests were originally added in
c859600954d ([PATCH] read-tree: save more user hassles during
fast-forward., 2005-06-07).

Since the exit code was ignored we'd hide errors here under
SANITIZE=leak, which resulted in me mistakenly marking these tests as
passing under SANITIZE=leak in e5a917fcf42 (unpack-trees: don't leak
memory in verify_clean_subdirectory(), 2021-10-07) and
4ea08416b8e (leak tests: mark a read-tree test as passing
SANITIZE=leak, 2021-10-31).

As it would be non-trivial to fix these tests (the leak is in
revision.c) let's un-mark them as passing under SANITIZE=leak in
addition to fixing the issue of ignoring the exit code.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>