REGTESTS: set_ssl_server_cert.vtc: remove SSL caching and set as working

REGTESTS: set_ssl_server_cert.vtc: remove SSL caching and set as working

In a previous commit this test was disabled because I though the
feature was broken, but in fact this is the test which is broken.

Indeed the connection between the server and the client was not
renegociated and was using the SSL cache or a ticket. To be work
correctly these 2 features must be disabled or a new connection must be
established after the ticket timeout, which is too long for a regtest.

Also a "nbthread 1" was added as it was easier to reproduce the problem
with it.