health_check_using_rpc: change _health_checking_using_rpc to _app_level_health_checking & use Address in OnHealthCheckRPCDone

The previous test certificates use 1024-bit RSA keys and SHA-1 for
signatures. They are somewhat deprecated. This commit updates the
certificates with RSA-2048 and SHA-256.

MesaLink requires an SNI set in client session. This commit sets the SNI
in the test cases. Note OpenSSL is not affected by this change.