open_stream takes its deadlines as parameters so the tests can use short
ones. The header-write test connects to a peer that holds the connection
without reading and sends a header larger than any loopback buffer; the
connect deadline is tested through the shared bounded helper with a
future that never completes, since a connect that hangs is not
reproducible without the network. The module-eval case asserts the
unit's RuntimeMaxSec and that the receiver script reads its header with
a timeout.
The socket is wanted by sockets.target and nothing orders the wireguard
unit before it, so the mesh address can be missing at bind time
(EADDRNOTAVAIL) and the receiver never listens. FreeBind lets the bind
succeed regardless, and also covers the interface coming back after a flap.
Also assert the mesh address is not a wildcard, which the comment above the
socket already claimed, and cover the rendered socket in a module-eval case.
