chore: add test for coord rolling restart (#14680)

Closes https://github.com/coder/team-coconut/issues/50

---------

Co-authored-by: Ethan Dickson <ethan@coder.com>

coderd/tailnet.go

@@ -75,7 +75,8 @@ func NewServerTailnet(
 	// given in this callback, it's only valid while connecting.
 	if derpServer != nil {
 		conn.SetDERPRegionDialer(func(_ context.Context, region *tailcfg.DERPRegion) net.Conn {
-			if !region.EmbeddedRelay {
+			// Don't set up the embedded relay if we're shutting down
+			if !region.EmbeddedRelay || ctx.Err() != nil {
 				return nil
 			}
 			logger.Debug(ctx, "connecting to embedded DERP via in-memory pipe")