certs: Break circular dependency when selftest is modular

The modular build fails because the self-test code depends on pkcs7
which in turn depends on x509 which contains the self-test.

Split the self-test out into its own module to break the cycle.

Fixes: 3cde3174

 ("certs: Add FIPS selftests")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>