certs: Add FIPS selftests

Add some selftests for signature checking when FIPS mode is enabled.  These
need to be done before we start actually using the signature checking for
things and must panic the kernel upon failure.

Note that the tests must not check the blacklist lest this provide a way to
prevent a kernel from booting by installing a hash of a test key in the
appropriate UEFI table.

Reported-by: Simo Sorce <simo@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Herbert Xu <herbert@gondor.apana.org.au>
cc: keyrings@vger.kernel.org
cc: linux-crypto@vger.kernel.org
Link: https://lore.kernel.org/r/165515742832.1554877.2073456606206090838.stgit@warthog.procyon.org.uk/