t5300: move --window clamp test next to unclamped

t5300: move --window clamp test next to unclamped

A subsequent commit will change the behavior of "git index-pack
--promisor", which is exercised in "build pack index for an existing
pack", causing the unclamped and clamped versions of the --window
test to exhibit different behavior. Move the clamp test closer to the
unclamped test that it references.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>