Add testcases for SSL_key_update() corner case calls

Add testcases for SSL_key_update() corner case calls

Test that SSL_key_update() is not allowed if there are writes pending.
Test that there is no reset of the packet pointer in ssl3_setup_read_buffer().

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16085)