swarm-controller: let each test nats-server bind its own port

TestNats picked a port by binding 127.0.0.1:0, dropping the listener
and handing the number to nats-server. Until that server bound it, the
port was free for a concurrent test's bind(0) to return again. Both
servers were then started on it (with the same store dir, since that was
keyed by pid and port); the loser exited silently and both tests'
clients reached the winner. term_stream's tests all use the stream
term-main-iris, and one of them creates it with max_message_size 4096,
so a test sharing its server got "stream name already in use with a
different configuration" (10058) from open_or_create's
get-then-create.

The server now binds a port of its choice (-p -1) and announces it in a
ports file, and each server has its own tempdir for the store and that
file, so a test can only reach the server it started.

Fixes #5261

Cargo.lock

@@ -4880,6 +4880,7 @@ dependencies = [
  "swarm-matrix-client",
  "swarm-queue-client",
  "swarm-secret-client",
+ "tempfile",
  "tokio",
  "tower",
  "tracing",