tests: Prevent testing dev_firmware* if the keys do not exist.
tests: Prevent testing dev_firmware* if the keys do not exist.

In CL:378661 we removed dev_firmware* from tests/devkey but that also makes
futility unit tests to fail.

This changes signing test scripts to first check if dev_firmware* keys exist,
and only use it (and test ZGB signing results) if available.

BRANCH=none
BUG=chrome-os-partner:52568,chrome-os-partner:56917
TEST=make runfutiltests; make runtests;
     add dev_firmware* back; run tests again and success.

Change-Id: If42c8404baf183edf5c8dbeadf537efa8ad571ec
Reviewed-on: https://chromium-review.googlesource.com/381151
Commit-Ready: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Tested-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Reviewed-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot