[test] Use mock random data for public key self-tests

Make the public key self-tests fully deterministic by temporarily
overriding the function used to obtain random data for RSA encryption.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/rsa.h

@@ -58,4 +58,6 @@ struct rsa_digestinfo_prefix {
 
 extern struct pubkey_algorithm rsa_algorithm;
 
+extern int ( * rsa_get_random ) ( void *data, size_t len );
+
 #endif /* _IPXE_RSA_H */