test: add rsa_verify() unit test

In this patch, a very simple test is added to verify that rsa_verify()
using rsa_verify_with_pkey() work correctly.

To keep the code simple, all the test data, either public key and
verified binary data, are embedded in the source.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>