tests/chip.c: Refactor to use default_layout from flash context

Default_layout is always available in flash context, and tests
need just one layout.

This makes test code a bit shorter, because now each test function
doesn't need to declare its own custom layout and set it to flash
context. default_layout is already declared and is already in the
context.

Change-Id: I1df459f1a0c27b5db8275a7840802c3d33c06fc0
Signed-off-by: Anastasia Klimchuk <aklm@flashrom.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/93400
Reviewed-by: Abdelkader Boudih <coreboot@seuros.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>