[Test] Hoist buffer default test imports (#642)

Follow-up to the #637 nit: the four buffer-default seam tests each
re-imported vllm_metal.config and vllm_metal.platform inside the test
body. Hoisted both to the module top.

Signed-off-by: RickyChen / 陳昭儒 <ricky.chen@infinirc.com>