Fix TlsPreferTest hang that aborts the Core2 test run (#1246)

* test: hold fake TLS server socket open to test end

The fake server answered the client's PING with PONG and then broke out of
its loop, disposing the socket. The client reconnects by default, so a close
that lands before ConnectAsync returns sends it back around its loop, where
it sees Reconnecting rather than Open and waits on a connection the listener
can never accept, its single accept being spent. Nothing bounds that wait, so
the test hangs until the session timeout aborts the assembly.

* test: report long-running tests in CI output

An assembly aborted on the session timeout prints no per-test output, so the
test that hung is unidentifiable from the logs. LongRunningTestSeconds is
delivered as a diagnostic message and does nothing on its own.