test: vboot: add tests for multiple required keys

This patch adds vboot tests to verify the support for multiple
required keys using new required-mode DTB policy.

This patch also fixes existing test where dev
key is assumed to be marked as not required, although
it is marked as required.

Note that this patch re-added sign_fit_norequire().
sign_fit_norequire() was removed as part of the following:
commit b008677d ("test: vboot: Fix pylint errors").
This patch leverages sign_fit_norequire() to fix the
existing bug.

Signed-off-by: Thirupathaiah Annapureddy <thiruan@linux.microsoft.com>
Reviewed-by: Simon Glass <sjg@chromium.org>