tls: Add test for blocked TLS handshake
tls: Add test for blocked TLS handshake

Add a test to ensure that we can't wedge the cockpit-tls server by
blocking during the handshake phase (which could even theoretically
happen by accident if a connection is dropped, for example).

This was a bug before moving to the threaded connection model.  The
lock-up eventually cleared when hitting the gnutls handshake timeout,
but 40 seconds is a long time for the server to be blocked from handling
any other incoming requests.

Closes #12674

cockpituous/cockpit,deryni/cockpit,cockpituous/cockpit,mvollmer/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,garrett/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,garrett/cockpit,deryni/cockpit,mvollmer/cockpit,cockpituous/cockpit,garrett/cockpit,garrett/cockpit,deryni/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,deryni/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,deryni/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,deryni/cockpit,cockpituous/cockpit,cockpituous/cockpit,deryni/cockpit,garrett/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit