Convert rsa_test to C++.

In doing so, check for malloc failures and use scopers as appropriate.
This should clear rsa_test for use with malloc tests. Also replace the
SetKey macro and exploded RSA keys with a DER RSAPrivateKey structure.
Much less tedious.

Change-Id: I3ce092ef67e7ac2af74f509abbdf84b7f2b6d45d
Reviewed-on: https://boringssl-review.googlesource.com/5043
Reviewed-by: Adam Langley <agl@google.com>