servertest: use memnet networking and add WithNodeExpiry option

Replace httptest (real TCP sockets) with tailscale.com/net/memnet
so all connections stay in-process. Wire the client's tsdial.Dialer
to the server's memnet.Network via SetSystemDialerForTest,
preserving the full Noise protocol path.

Also update servertest to use the new Node.Ephemeral.InactivityTimeout
config path introduced in the types refactor, and add WithNodeExpiry
server option for testing default node key expiry behaviour.

Updates #1711

hscontrol/servertest/ephemeral_test.go

@@ -12,6 +12,24 @@ import (
 
 // TestEphemeralNodes tests the lifecycle of ephemeral nodes,
 // which should be automatically cleaned up when they disconnect.
+//
+// TODO(kradalby): These tests wait for real-time grace periods and
+// GC intervals (up to 60s). testing/synctest would allow instant
+// fake-clock advancement, but three blockers prevent adoption
+// as of Go 1.26:
+//
+//  1. zcache janitor goroutine: No Close() method; stopped only via
+//     runtime.SetFinalizer which runs outside synctest bubbles.
+//     - https://github.com/patrickmn/go-cache/issues/185
+//     - https://github.com/golang/go/issues/75113 (Go1.27: finalizers inside bubble)
+//
+//  2. database/sql internal goroutines: Uses sync.RWMutex which is not
+//     durably blocking in synctest, causing hangs.
+//     - https://github.com/golang/go/issues/77687 (mutex as durably blocking)
+//
+//  3. net/http server goroutines: I/O-blocked goroutines are not durably
+//     blocking, preventing bubble termination.
+//     - https://github.com/golang/go/issues/76608 (httptest synctest support)
 func TestEphemeralNodes(t *testing.T) {
 	t.Parallel()