Let's use RSA_3 rather than 3

Stop setting RSA_FLAG_SIGN_VER

With rsa_sign.c r1.37 this is no longer needed.

rsa_method_test: some consistency tweaks

fix typo

Add some regress coverage for custom RSA methods

This currently only covers sign and verify since other parts are already
known to work in practice. Prompted by a bug report by kn