TestBmpBlock: use real tempdirs instead of $PWD
TestBmpBlock: use real tempdirs instead of $PWD

This lets us do proper out-of-tree builds by not writing to the cwd.

BUG=chromium:466499
TEST=precq still passes
TEST=out-of-tree ebuild passes
BRANCH=None

Change-Id: I49eb013cf9c2f5ff73cfab155612aaa4d1d724cd
Reviewed-on: https://chromium-review.googlesource.com/259520
Trybot-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

acorn-marvell/vboot_reference,acorn-marvell/vboot_reference,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,acorn-marvell/vboot_reference,coreboot/vboot,acorn-marvell/vboot_reference