Mitigate a race in test_HTTPSConnection_SNI_support

On ppc64/ppc64le, test_HTTPSConnection_SNI_support has been failing with

AssertionError: 'Hostname in TLS extension: "localhost"' not found in 'Setting secondary ctx parameters\nUsing default temp DH parameters\nACCEPT\n'

i.e. the connection-specific data has not been logged by s_server.

Wait before terminating it to allow it to flush its buffers.