testing: Encryption with multiple keys

[phish108]

Multiple keys require key wrapping alg. (Eg not ecdh-es but ecdh-es-...)

• test for RSA and EC keys
• test for multiple recipients sane headers
• Test for multiple recipients with differen headers
• test for multiple recipients with different key types and headers
• verify that mixing key types is not allowed.

[phish108]

This is only feasible with the encrypt tool if all recipients use the same keytype and alg.