faft: Skip checking GBB flags after the first test finished

There is no need to set GBB flags at the beginning of every test. We
just need to set them when running the first test. This saves us about
8 seconds per test.

BUG=chromium-os:34392
TEST=Run a test suite. Check GBB flags is only set in the first test.

Change-Id: Ib23e1326d208634e7331ff2bb54000529d11afb0
Signed-off-by: Vic Yang <victoryang@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/35811
6 files changed