Increase timeout on gnutls-serv for slow tests

[gilles-peskine-arm]

We sometimes want to run TLS interoperability tests with slow crypto and with a sanitizer such as Valgrind or MSan that slows down execution. (Another candidate could be CPU emulation or a slow piece of hardware, but we currently don't have that on our CI.) When we test interoperability against a GnuTLS server, we may run into its 30-second timeout. This happened in #9742.

GnuTLS ≥3.8.1 allows configuring the timeout on the command line. The goal of this issue is to use this instead of disabling the test cases, which is how we resolved #9742 in a hurry. Note that since this requires a recent GnuTLS, it is not applicable to tests that use an older version. In particular, at the time #9742 happened, even the most recent GnuTLS in our test environment was too old.