t1450: check large blob in trailing-garbage test

Commit cce044df7f (fsck: detect trailing garbage in all object types, 2017-01-13) added two tests of trailing garbage in a loose object file: one with a commit and one with a blob. The point of having two is that blobs would follow a different code path that streamed the contents, instead of loading it into a buffer as usual. At the time, merely being a blob was enough to trigger the streaming code path. But since 7ac4f3a007 (fsck: actually fsck blob data, 2018-05-02), we now only stream blobs that are actually large. So since then, the streaming code path is not tested at all for this case. We can restore the original intent of the test by tweaking core.bigFileThreshold to make our small blob seem large. There's no easy way to externally verify that we followed the streaming code path, but I did check before/after using a temporary debug statement. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>